building swaping between reachable and unreachable state

I encounter a strange behavior.

The swapper under construction swaps between reachable and unreachable state when the buffer gate just above change state :

When the buffer gate output is active, the swapper is reachable.

But as soon as the buffer output becomes inactive, so the swapper goes unreachable. O_o

There's no doors connected to this automatism system.

And if I deconstruct the granite tile on the right of the swapper, it stays reachable.

Astraea.sav