The only “solution” I found so far was to hack rtaudio and replace the relevant hard-coded 48000 with 44100 and then compile the Rack myself.
But … in Rack v1 rtaudio had the same hard-coded 48000 values and it works fine with Rack v1.
It seems that there’s some bug in the code that initializes rtaudio without setting the desired sampling-frequency rigth at the beginning.