Although Harmony is still not showing up in the new SmartThings app. I used the Classic app and followed the advise here to get the virtual activity switches to sync:
I’ve seen lots of complaints about how long the Harmony switches in ST take to update when using the physical remote to turn stuff on or off. Just found a really simple way to get instant state updates. Go back and add all of the ST switches to your Harmony hub as Home Control devices, then turn each corresponding one on when your activity starts and back off when it ends.
Bam, instant Harmony status updates.
Step by step below. This starts with the assumption that you’ve already added your Harmony hub(s) to SmartThings.
Open the Harmony app on your phone (as far as I’m aware, the phone app is the only way to add/edit Home Control devices)
If you have multiple hubs, connect to your target hub. These steps have to be repeated for each hub.
Add your SmartThings switches to Harmony…