nugluglu I have already tried disabling the sensors permission for Molly but this didn't seem to affect this.
other8026 have you disabled the app's sensors permission?
Wow. I didn't see you say that the first time. Sorry for not being able to read!
I was curious about this so I did some searching through Molly's code, and it looks like Molly uses the proximity sensor only when playing back audio "notes", aka voice messages.
It looks like Android apps that offer voice calling have to use an Android system component (Telecom), so, unless I missed something, the only other real possibility is it's managed or set by some system app or service. And since an OS component is handling that part of the call, disabling sensors won't do anything.
You can try that by sending an audio message to yourself and holding your hand near the proximity sensor, then grant Molly the sensors permission and try again. You'll find that the proximity sensor is affected by the sensors permission as you'd expect.