Not all Schengen countries stamp passports of residence permit holders; this depends on national practice. Germany (along with, for example, Switzerland and Slovenia) doesn't stamp passports of residence permit holders.
And being that you have a valid residence permit, that is what dictates how long you can stay, so you will have no problems exiting or re-entering Schengen in the future.
As for the UK, they're not a Schengen country, and so don't care about what Schengen stamps you have or don't have.