Forging a lease for an empty dwelling is easy. But forging a lease for the owner's residence is much harder. You'd have to get rid of the owner's stuff, make sure the neighbors don't testify that this is the owner's residence, come up with fake papers to prove the owner lives elsewhere.

All in all, i'd be curious if you have an example because that sounds like opinion manipulation from the owners as that case is very unlikely. At least similar cases i've heard about in France have been 100% debunked. It's much more likely the owners live(d) elsewhere and had an empty dwelling squatted.