Have you been through the list I suggested above?
No developmental concerns doesn't mean that there are definitely no flags at all - it could be that there are low level things across the board that individually aren't triggering anything but when taken all together are just enough to say yeah OK, something is going on here, let's take a closer look.
It does sound like you're unhappy with the nursery, so it might be worth seeing if there are vacancies anywhere else but you keep saying "I don't know what they can do" when people have suggested lots of things that they can do.
Basically with behaviour you can look for potential causes, which include sensory/attention/development/dysregulation, you can adjust the environment, you can adjust the demands, sometimes temporarily while you teach skills, and you can respond directly to the behaviour (which includes everything from reasoning to redirection to emotion coaching to rewarding alternative behaviour to punishment).
So there's loads which can be done but it does need a discussion between as many of the adults involved as possible and a plan of action.
I know some nurseries can be a bit crap on this front - ours were and we did end up moving DS, but there were other (multiple) issues with the nursery, they were interested in trying to explore it with me and we had come up with some ideas for things to try.