Hi @Christine_Bolton, good question! We have a similar question on this topic, adding here for visibility: Limit Project Custom Field visibility so it's only visible to the project members (not collaborators)
There are a couple of things that some to my mind, could you confirm if the users are guests or members? and are the tasks where they can’t see the fields “subtasks”?
cc @ambforumleader @pforumleader in case you have any workarounds to share with Christine!