Thought I'd provide the findings in the event anyone may benifit.

Turns out the protective cover managed to pull out of the retainer clip. It appears the issue may be that original case is hard and the little detents in the clip are struggling to embed themselves into the case.

I have it redone and seems to be holding at the moment (with full travel of the heater control)

In the event this happens again, is there anyone that makes reproduction retainer clips?
And is there an easy way to get these off (STRUGGLED to get that thing off)