THE PROBLEM
Beginning in mid 2020, some users reported that presence on iOS phones and tablets no longer worked reliably to trigger smartthings routines because the smartthings app wasn’t always open in the background. Typically as soon as they opened the smartthings app it would work again.
At the same time, if they were just using the phone’s presence in native Apple features, including HomeKit, presence worked fine. and sometimes ST presence would work fine for one person in a household but not another, even if they had the same model iPhone. 
Hopefully eventually the two companies will get this straightened out and iOS presence will work universally in smartthings again, but in the meantime, if you want to, you can use a physical device which is visible to both HomeKit and smartthings as a proxy for presence.
Which device to use?
2024 update
Sadly, like some others in the community, the Meross cloud to cloud integration, which had been reliable for me for several years, stopped changing state correctly. I have contacted both companies, no resolution. So I am going to leave the information about Meross here for now in the hopes that this problem gets fixed, but meanwhile, if you do have a SmartThings/aeotec hub, and can use Matter, an inexpensive matter. Plug might be a more reliable option for now. If you don’t have an ST hub, you can’t use matter, so maybe something attached to a Hue bridge or another cloud to cloud option that shows up in both platforms.
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This problem appears to be specific to smartthings: The Meross devices have no problem synching state in multiple other platforms. It’s very disappointing.
Personally, I like Meross devices and use a number of them. These are Wi-Fi and work well simultaneously with HomeKit and smartthings. They have good safety certifications. And they are relatively inexpensive.
You can just use one of their mini plugs for each person that you want to track. But I am particularly fond of their power strip for this purpose. You can get either a three socket or a four socket one, each socket independently controlled, so that lets you represent up to four people in the household from just one wall outlet. They also have USB charger slots which you can use even while using the sockets for presence. And if you turn the socket on when you are home and off when you’re away, that might even be useful with something plugged in, it just depends on your needs.
These are available in both the US and the UK.
Make sure it says HomeKit compatible: they have a slightly cheaper line that isn’t, but you need HomeKit for this project.
US VERSION
At the time of this writing, the four socket strip cost just under $35 but there are sometimes additional discount coupons available on the page.
https://www.amazon.com/meross-Smart-Power-Strip-SmartThings/dp/B07NBNJ32V?ref_=ast_sto_dp
UK VERSION
https://www.amazon.co.uk/meross-Compatible-HomeKit-Protector-SmartThings/dp/B08JG232D8/
Create automations in HomeKit to turn a socket on and off
Homekit Automation
If that works well, you’re on your way. 
Now you can use that socket as a proxy for presence for that person
Since both HomeKit and SmartThings can recognize the Meross at the same time, you can just use the socket as a proxy for presence: if the socket is on, the person is home.
SmartThings routine
ANY PLUG THAT IS VISIBLE TO BOTH HOMEKIT AND SMARTTHINGS WILL WORK FOR THIS, INCLUDING MATTER DEVICES
As I mentioned, I was already using the Meross devices and like them, so that’s what I’m using. But there are several other brands which can do the same thing, particularly now that matter is available. So what you choose is up to you, it’s all the same method. Add a device to both platforms, create a HomeKit automation to turn it on when you get home and off when you leave, then create routines in smartthings, for when that plug turns on and off.
Some other choices:
If you have a hue bridge, there is a hue plug that will be visible to both platforms. Like Meross, this method does not require a smartthings/Aeotec hub.
If you do have a SmartThings/Aeotec Hub, Tapo makes a very inexpensive UL listed Matter smart plug:
https://www.amazon.com/TP-Link-Tapo-Supported-P125M-3-Pack/dp/B0BNWGZ545/
The good news is that matter integrations are local, so as long as HomeKit knows you are home, you should be able to make your smartthings routines local.
The bad news is that your Wi-Fi router does need to support IPv6 addressing, which you don’t need for the Meross cloud to cloud integration.
FAQ: What do I need to add a Matter device to the SmartThings app? Do I need a bridge router device?
Choice is good.
Virtual Presence Sensor
If you want to get fancier, and you have a SmartThings/Aeotec hub, you can create a virtual presence sensor and turn it on with an ST routine when the socket is on. This then allows you to use “member location“ in smartthings routines and to do things like say “if everyone is away.“
Here’s a community-created Edge driver for a virtual presence sensor.
[ST Edge] vEdge Creator: a virtual device generator for end users
That’s your choice, but for most people just the socket should be enough.
Other issues
All of this is just a method for letting you use the phone’s presence in ST without requiring the ST app to track presence since it doesn’t seem to be very good at it for some people.
If HomeKit presence isn’t reliable either, then that’s a different issue and would require a different solution.
IF YOU HAVE A SMARTTHINGS/AEOTEC HUB, A MATTER SMART PLUG SHOULD BE LOCAL TO BOTH PLATFORMS
The cloud to cloud Meross integration to Smartthings broke for a couple of weeks in 2023, then got fixed for a while, and then broke again in February 2024. 
If you prefer to keep everything local and you have a smartthings/Aeotec hub, you can use any matter-compatible plug since that should work with both platforms. Tapo makes an inexpensive one, but we should start to see more over the next few months.
If you don’t have a smartthings/Aeotec hub, you can use the hue smart plug that works with the hue bridge.
