HI Climbee, that is not true sorry the installer is telling you fairy tales. It actually makes sense. Take a look at the water flow/connection of the I-Box and then you will see that he is telling you fairy tales.
The installation is correct, but as I said, there is one valve (namely the one at the thermostat for upstairs) too many.
If it doesn't bother you, just leave it and forget my comments about it. The world won't end because of it. I just wanted to make you aware of it because it would bother me if I always had to press two buttons when I want water from upstairs.
We have a panel as a shower wall, I carried it up myself and it is free to be viewed on three sides. There are no such bubbles on any side, nor on the one that is now embedded in a thin U-profile in the wall. In my eyes, this is simply poor workmanship, the edges on top also do not look quite clean on the slopes.
Edit: True words from borderpuschl, if it suits you that way, then I wouldn't stress about it.
So, I had inquired at a local glazing company whether this is normal with laminated safety glass.
Just now the friendly owner called me back (which I did not expect at all): no, that is not normal, file a complaint immediately!
Good to know: these bubbles enlarge over time - slowly, but they do get bigger (now I also know why they are so large here in the office glass windows - the building is a good 20 years old). So they would eventually no longer be covered by the handrail.
By the way: yes, the handrail is mandatory in Germany and is always supplied and must be paid for - whether you then put it on or not is no longer the glazier’s business.