\autoBeamOff when used with
\partCombine can be difficult to understand.
It may be preferable to use
\set Staff.autoBeaming = ##f
instead to ensure that auto-beaming is turned off for the entire staff. Use this at a spot in your score where no beam generated by the auto-beamer is still active.
Internally, \partCombine works with four voices – stem up single,
stem down single, combined, and solo.
In order to use \autoBeamOff to stop all auto-beaming when used
with \partCombine,
it is necessary to use four calls to \autoBeamOff.