Summary
I use a layout of 3 screens in macOS Catalina, and I also make use of multiple "spaces" per each window, instead of using apps in "full-screen" mode. I noticed that when I did an "OPT+Click" on the "zoom/fullscreen" button in the top bar, which is a shortcut to make the window expand to take up the full space, the window zoomed to the top instead of taking up the screen the window was on. To summarize:

Expectation: Doing a window "zoom" action makes the window take up the full screen of the current screen that the app window is displayed on.

What happened: App window changed size to match the screen size, but moved to a completely different screen instead.

To see this bug in action, please see this video.