Stalling can be fine if the horse has daily turnout and plenty of grass hay to munch on in their stalls. However, I agree, stalling for more than half the day can cause some issues. The biggest issue is plain old boredom. If a horse gets bored, they can start behaviors such as pacing, weaving, cribbing, wood chewing, pawing, etc. Having lots of hay around will usually help alleviate boredom for most horses. If you cannot increase hay, you can use small mesh hay nets to make it take longer for the horses to eat their hay, or feed the horses more often during the day but with smaller meals.
If the barn is reseeding their pastures, they should have used electric and split them up, so they could rotate the horses from pasture to pasture as the reseed. That would have left the horses out, with just less room.