Hello Vadim
I apologize for the late reply. I’ve gone over this one with Rene and the behavior is indeed what is expected. Briefly stated we have to think about the whole MST topology as a single switch. PVST+ perceives SW2 and SW3 as a single switch, and as we know, each switch can have only one root port. If SW3 perceived SW1 as the root, it would have to create a root port on Gi0/0. But that means that the “MST switch” would have two root ports (the other one being Gi0/0 on SW1) and this is not allowed.
Rene has actually gone in and revised the lesson to include this explanation in detail.
I hope this has been helpful!
Laz