You know, you are coming across as very crotchety, folks.
So. The box has a Netflix app in it. Netflix master their own shows in 24fps (actually 23.976) and/or 60Hz.
Therefore it is very likely that Netflix, and possibly Amazon Prime Video material will be mastered and streamed in a refresh that is incompatible with the 50Hz.
Now, is it a "lock" or not? I'd suggest a lock is a thing that has a key, in this case the metaphor is that the lock could be unlocked by a software/firmware 'key'.
Anyway thanks for the critiques of my use of using the term 'lock', which by the way was from the review on AV Forums here:
https://www.avforums.com/review/humax-fvp-5000t-freeview-play-pvr-review.14383
@grahamlthompson does make the most useful comment, in that he states that the broadcom chip used doesn't support 24fps, although he doesn't state if it supports 60hz. They then, sadly, ignore the fact that I was asking with regard to Netflix materials on the appropriate app, and not Freeview-HD.
That suggests it's a hardcoded chip capability.
So the question has, at last, probably, with lots of lovely snipes at my semantics and motivations, that it's unlikely to cope with 24fps or 60hz materials natively on Netflix.
Which oddly enough, when you read my OP, is *exactly* what I asked.