Sounds to me like the “go away” diagnosis. They can’t figure it out so they tell you it needs a new engine. One of two things results- you agree to replace the engine. Yay! We get a big job for lots of dollars and the pesky problem goes away with it. Or 2, you go away and their problem is solved.
Time for a new shop to have a go at it. Hopefully, they are better diagnosticians than these guys.