There are a couple of bugs that come together here:
* Particle hacks: extra modifier stack evaluation just for particles in
rna_Object_create_duplilist. This is where the primary issue stems from,
the "for_render" setting replaced the G.is_rendering flag in threaded
depsgraph. This causes particles to recalculate the entire modifier
stack with _render_ settings instead of viewport settings now. Fixed by
taking the 'preview' parameter in Cycles into account.
* Buggy skin modifier: The skin modifier generates a different amount of
vertices and faces **on every execution**. This must be looked at
separately, but it could be another reason why cycles constantly
restarted the sync process.
* Particles get re-distributed randomly every time (changing seed). This
could be caused just by the broken skin modifier, but might still be an
issue when simply rendering with cycles, since the psys will be
evaluated for render settings, if just temporarily.
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