Hello, it seems that it solved your case. But since i have a similar one i will reply also here.
Long story short my magikarp wont evolve even after countles levels the evolution srceen does not show up.
So i tried what hahahaka suggested but it does not work for me so far i tried it even with Rare Candys while it was down and the happiness was below 255. But no effect.
After this tried a second lvl 5 Magikarp an leveld it to 20 same result no evolution screen appearing i fear that this is maybe an larger problem. I tried only Magikarp so far. So i dont know if can evlove other Pokemon so far. But i will try. Earlier this day everything was it seemed to ocurr after the recent shutdowns and rollbacks.