I can’t replicate this with the same phone, os version, and app version, but I suspect given the description, and that this occurs on every iOS device I’ve done the specific things to replicate, you may be experiencing this issue I’ve reported here:
If it’s the same issue I experience from time to time, reinstalling the app won’t work.
The only reliable fix I’ve found is to completely quit the app and force an iPhone reboot. To do this on your phone quickly press volume up, then volume down, then hold the sleep/wake button until the screen goes black and continue to hold it until you see the apple logo.
Once your phone powers back on and connects back to Wi-Fi there’s a good chance the app will work again. Good luck!