I think they showed a panel of them surveying the surroundings.
It's simply as they said, Macht's mana makes it impossible to detect anything else once you get close to the barrier. And Solitär used that opportunity while Frieren was distracted to make her move which was the only time they could've noticed.
From my understanding, the barrier didn't crumble over time, rather Solitär analyzed it for those two months to be able to lift it then.
Can I say, I feel bad for Stark's only idle activity being push-ups apparently. That guy's with Frieren who takes breaks for months regularly and all he gets to do then are push-ups