When utilising Alt+Tab and in a fullscreen borderless game like Escape from Tarkov and League of Legends, pressing alt+tab will instantly close the dialogue. This does not occur in Overwatch.
When holding Alt down, it will continously flicker. Sometimes when spam clicking to alt tab it will randomly get fixed. I have attached a video of what happens when holding down alt using Current Monitor + Current Windows.

So far, the only configuration in which this does not occur is Show on All Monitors, Show Current Windows Only in which it will close the first time, but will work on the second attempt and beyond but this behaviour resets when you tab out. In this mode however, you cannot change the selected window with Tab.

Sometimes, I have been able to fix this by spamming the alt+tab or by changing to fullscreen and back but it is not replicable nor does it happen often.

This behaviour happens on 12.0.2 and 12.1 Beta v3