It can happen that you think your oscillator is tuned to the correct scale, because some notes of the scale would overlap with the scale you think you set. With more notes appearing you might notice it’s not the correct scale but interpret it as “Marbles switched scales somehow”.
a) Tune your oscillator to the desired root note, after that connect with Marbles’ x out.
b) Connect Marbles’ x out to your osc. Set Steps fully CW (so only the root note and octaves will we played), set Spread to CW or CCW (so the notes won’t change on the output). Set x-Bias to CCW or whatever position you feel comfortable tuning your oscillator with.
Tuning your oscillator will determine the root note. The respective scale is set via longpress Output Range.