If I put the valve with the outlet up, there are continuously micro bubbles that go into the pipe and finally agglomerate into big bubbles.

Yes, I put the solenoid in the housing (contact between the square edge and the housing).
I mounted the solenoid with oil (10w40 for car engine).

I agree with your explanation about the shock wave created when the valve turns, but is it surprising that I am one of the only guys to have this problem?

I use the fuel of my lawn mower, 95E10 + 10% synthetic 2-stroke oil (I think this fuel is 3 months old)