Occasionally, the player will be unable to interact with items in the creative inventory E.g, I was gathering "natural" blocks in my hotbar, filling all 9 spaces & began using them. When I went back in to the inventory to add more blocks, clicking on any inventory space (in either the player inventory proper or the block palette) appeared to do nothing. Exiting & reentering the inventory GUI worked as a workaround.

Anecdotally, some have said this occurs in survival as well. I wasn't able to reproduce anything.