They are protected when they are in the fortress, but any previous reductions are permanent. You would have to play a heal card to return them to normal strength or remove any lingering effects.
For instance:
Your 10/10 hero is being reduced 0/2 per turn.
After your second upkeep phase, your hero is now 10/6.
You put the hero in Goshen, so there are no more reductions, but your hero is still 10/6.
You now remove the hero from Goshen as a 10/6 hero.
On your next turn, during your upkeep phase, your hero drops to 10/4.
This continues until your hero reaches 10/0 and is discarded, or your hero remains in Goshen before reaching 10/0, or you play a heal card.
