Same problem on S9+. Defeats the purpose of Work Profile imho... I was not able to enable Gboard, Swype or SwiftKey. I'm demoing InTune for my company, and by default there are no device restrictions, especially nothing about third-party keyboards or settings, etc. So it seems to be Knox-specific, with no way to actually control those settings that I've found yet.
Side note: deleting the work profile (that is, removing the Company Portal app, which removes the rest) restored my ability to select my own keyboard.
Now, I know keyboards can collect whatever data you type... but I specifically allowed Gboard (for example) as an approved app within the Microsoft InTune admin center, and I *still* couldn't enable it.
Samsung: please explain.