Well, that’s quite strange indeed. Here’s some more information.
I attached two Dorico files, both with just one B natural whole note. At least in my copy of Dorico, one of them shows C sharp in the clarinet part, and in one of them it is missing.
Both times I entered the note in score, and then went to the part to see if it worked. The only difference is in if I applied the natural before or after creating the note. If I enter the natural first by typing 0 and then B, it works: I can see a C sharp in the clarinet part. But if I first enter B, then select the note, and then type 0 to add the natural, it doesn’t work.
Also, in the not-working example, if I explicitly set the accidental to be shown in the properties panel, like so:
Then the accidental does become visible in the part also, but it is a natural - and not a sharp, as it should be:
Missing accidental and working version.zip (649 KB)