Hi Kevin,
This is a well known error in mendix.
This happens when you commit the parent object then all the associated objects and associations is also auto commited and saved in the database to keep the model in sync.
This is done by mendix ( but the associated objects are in changed state).
So,
You need to explicilty commit the children/associated objects in the microflow too.
for more info read this documentation.
https://docs.mendix.com/refguide/committing-objects/#autocommit-and-associated-objects.
Hope it helps 😊