I am using the Microsoft Teams app from the Google Play store, on a Google Pixel 4a. Everything is up to date, and this issue has been happening for a few weeks at least.

When a voice call is open, whether in the background or on the foreground, the entire screen turns black under specific circumstances.
If anything (hand, finger, head, bottle of water...) is directly in front of the top right corner of the screen, the screen turns black. The exact spot it happens is around the wifi icon, up to 5 centimeters away. If the item is moved, the screen turns on again in less than a second.

This issue seems to be similar to how the screen will turn black during a regular phone call when the phone is at the ear, but it will turn on again when the phone is moved away from the head. It prevents the ear/cheek of the user from accidentally pressing a button when the phone is at the ear. Unfortunately, it makes it very difficult to use the phone if this happens during a Microsoft Teams call, especially with the right hand always going over the "sensor" area.

The only way to prevent this is to use the "speaker" sound setting instead of the "device" one, but this solution was only found after hours of looking for a solution and isn't intuitive.