That’s a bug with the SmartThings mobile app. I’ve noticed that if the connection isn’t solid (i.e low latency) it has issues in response and status. Try to reboot your phone or put it into airplane mode and then reconnect to the mobile network. A little trick about GSM networks, the last connection is the fastest, so as your connection gets older it get slower. It helps to reconnect to the network if you’re facing data connectivity issues.
Alternatively you can also check out more advanced user management apps like this one, it doesn’t rely on the mobile phone. So for example you have a recurring schedule or expiration date for a user you don’t need to manually log into the ST app each time and manage the users, you set it and forget it, it run in the cloud, so as long as your hub is connected to the internet it’ll do the rest for you.