Full screen application turns to windowed application and leaves a black screen in Mission Control
I found a temporary solution: This happens on Big Sur when applications open pop-ups as tabs in full screen mode. Closing those tabs seemed to have caused the issue as far as I am currently aware.
To fix this currently, I went to System Preferences &gt; General &gt; Prefer Tabs: &quot;never&quot;
This also fixes JetBrains IDEs (PHP Storm, Android Studio, PyCharm, IntelliJ IDEA, etc) becoming unresponsive with pop ups. After I found a solution to that issue I was having, it seemed to fix applications in full screen like Firefox and XCode having the primary issue I inquired about above.