I have also had issues with FL Studio: https://iplug2.discourse.group/t/lots-of-posts-on-this-forum-about-issues-with-fl-studio/488
I know you’re reporting a graphics issue but it also seems the VST “specs” in general are loosely followed by some DAWs (FL, Ardour, etc). How else do we explain plugins that work perfectly everywhere else except DAW “X”?
Unfortunately, that’s where the need for “if (GetHost() == such and such)” comes in. Some plugin manufacturers (like Fabfilter) must have a ton of such conditional coding because their plugins seem to work everywhere.