Yeah, I'm having the same problem. For secondary profiles, the default keyboard reactivates itself when I logout and login again. At this point, I can only take it as a bug in the system as, again, this is for secondary profiles only, and not for the main profile.
As a test, I may create one more profile, just to test if disabling the default keyboard app altogether actually does break anything. That would be the only workaround for the apparent bug for now. Under normal circumstances, no, it isn't a good idea to disable system apps. But under normal circumstances, users' preferences would still hold the next time they sign in. This isn't, or at least shouldn't be, a normal circumstance.