Hi @GarryPope,
This doesn't happen anymore. I haven't changed anything, so maybe Microsoft has fixed it?
I now have a slightly different issue.
My power app has a table of contacts with a line under each contact, to separate them. When a user opens Microsoft Teams, the window isn't maximised, which is normal. My power app will display the table correctly, see below:

But when Teams is maximised the table doesn't adjust properly, so the contacts don't sit within the boxes.
See the example below:

It goes back to normal once I search for a contact or click away from the app and then back again.
It's not a big deal just slightly annoying.
Thanks,
Liam