I think it was aspects like this that let Voyager down.
I've never served in the Navy before, but I doubt any officer is a Ensign for that period.
Single ship, no other ship to transfer to, only so many commissions to go around, no one above Harry died or transferred for him to be given their Title, and obviously there was no ship for Harry to transfer to for a promotion. It just wanted possible. But we were shown that once they got back to The Federation he was indeed promoted, at least all the way up to Captain eventually. Personally, I believe this is something they got right.
Or, you could choose to believe Captain Janeway just wanted to make the boy suffer.
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?