I am using i3 since many years and I have not come across an application that is non-compliant with the X windows controls such as Teams is, even when trying to close the window using wmctrl it does not work:
[thomas@idefix] wmctrl -v -c 0x03000005
envir_utf8: 1
Invalid type of WM_NAME property.
Invalid type of WM_NAME property.
Invalid type of WM_NAME property.
Invalid type of WM_NAME property.
This behavior is identical whether it's a floating or a tiled window. Not all programs are tiled window-aware but all close a window when instructed to do so. I'd say it definitely is a bug in Teams.
A note to the "On close, keep the application running" setting:
With the checkbox enabled he can terminate the software by right-clicking on the tray icon. Disabling the checkbox does not change anything because the user is still forced to keep the Teams window open if he wants to remain in touch.
The idea is to have Teams run in the background and inform the user of incoming communication which is currently (my version is 1.5.00.10453) not possible without having a window open.