So, Nintex is telling me it's not a bug, but has requested to see my workflows, and I've sent them along.
I am going to mark Marian as correct, but will update once I receive more information from Nintex on this issue.
the reason is explained here https://community.nintex.com/nintex-blogpost/1289%C2%A0
when workflow is started on a event, it can be started sooner then list item is fully instantiated. then it has these undesired side effects.