Been trying to solve this all day - I'm running Nvidia surround on 3 1080p monitors, using the Windows TB on the center and DF on the sides using the split monitors. Maximising a Windows on the center monitor will force the app behind the Windows taskbar, DF taskbars act as they should.
I've found numerous posts on this from years ago but none of the fixes seem to work. How is this still an issue?

Thanks for the reply Keith! Managed to solve the issue - I was looking at a post about renaming the nvidia nvsmartmaxapp but was confused as to why I couldn't find it in the location of the post. I searched my C drive for the file and found them in a different folder - renaming them solved the issue.
I'm guessing for whatever reason, DF wasn't detecting and asking to disabled the nvidia surround services.