GTK app only goes properly full-screen on the manually-installed desktop environment [multiple displays]

I have a triple display setup on Debian, and am using gtk_window_fullscreen_on_monitor() to display on two of the monitors.
A bizarre thing occurs where my default installed desktop environment (i.e. the one selected on tasksel during system installation) does not properly fullscreen one monitor and instead, one row of GTK Buttons overlaps into the second monitor and leaves open space under the app.

If I install a new desktop environment with APT and switch to that one, the full screen works properly. I have tried this with both 1) GNOME tasksel, manually install cinnamon and use that to fix the issue, and Cinnamon tasksel, manually install gnome-session and use that gnome to fix the issue.

Before I delve deeper, has this ever happened to anybody else? There must be some system setting that renders content differently from the default-installed desktop environment versus the manually-installed one. Or perhaps GTK reacts differently to the latter option.

It appears to be once again an issue with min-width and label wrap settings. In fullscreen mode, whenever there is an ‘overflow’ in the GTKBox row of GTK Buttons, GTK will bug out and not go properly fullscreen, and instead drag some of the screen onto another monitor. At least on GNOME. Cinnamon must render it a bit differently (e.g. smaller font sizes) and thus reacts differently to full screen overflows.