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.
I don't buy that. Tuvok got promoted, but still had the same position. Rank and position aren't the same thing. There were numerous promotions in TNG and DS9 in which characters had advances in rank and not position, and this was often done in recognition of good work and not for new positions elsewhere. I think it was even said once that officers are automatically promoted to Lt. junior grade after three years or so.
For his good work as an officer, that alone signals promotion-worthy. Just shows how the show was poorly managed lol..