Scale change

No, not a bug. It's called Artificial Intelligence ;-)

I don't exactly remember the code behind this right now, but scale selection is sensitive to the ordering of chords. Which scale is selected depends on the chords you played previously. So yes, if you click the same chord multiple times, a different scale might appear.

There is a large choice of potential scales for each chord in any key. Without melodic material as a limiting context, scale selection is largely a matter of taste and style. 

You can change scales in the Harmony parameter at any time and observe its effects. Effects are most likely to be noticed with rich melodic material.