eheh
Well
I may have an answer to why my top menus were vanishing
Y'see, I completely forgor that I added a Global Menu to my desktop panels, which will tell all applications that respect its presence (likely to be native and AppImages, but less often Flatpaks) to smack its menu bars up there.
The thing is, since I set that panel up on a third monitor and now I just have two, I forgot all about it. ...But KDE didn't. Kinda wish there was an indicator that KDE was doing that.
Something to keep in mind if you're wondering why your menu's are missing!