I’ve been having this problem in 5.1
It seems that if I change functions access specifier from private to public, that solves the problem.
If I change to private, I cannot duplicate or copy and paste a function. I have to drag it from the My Blueprint panel.
If I change its access specifier to public, then I can copy and paste as normal.
Note that this is a function defined in the same blueprint class, but it acts as if it were a private function in a parent blueprint. So it seems to be a bug.