I think I too encountered the same issue… made two topics about it with no answer (Please help me figure out what's going on)
but just now I had an idea and finally found what cause it: bendy bones. Rigify use bendy bones as default. The bendy bones segments move even when the octahedral form seems to be not moving.
what you can do: reduce the bendy bone segments to 1 on each bone or just visualize it by switching the display type