This is just logic rather than medical studies, but it would seem reasonable that if breasts disappeared routinely when estrogen dropped below female levels, no trans men would need full top surgery and cis men with gynecomastia wouldn't need reconstructive surgery either.
(Diminished somewhat, maybe, but the breast tissue doesn't disappear once it's been developed. I would bet that what happens is that the *fat* redistributes to male locations again, and "breasts" are actually a large percentage fat, so... plus boobs which are softer and flatter/more droopy will look smaller overall. If the actual breast tissue was still minimal, the chest probably would end up looking almost the way it did before.)