I have had customers who had problems with this type of rough-in and the handle slipping.
There is a way to fix it but you need some o-rings 2 of a #01 224 000.
Grohe should be able to send you no charge if you explain the situation.
I have the instructions on how to fix but it won't upload here.
You can e-mail me you address and i can e-mail it to you.