Yes, I have had this problem too. While working on a particularly large assembly (some 3500 parts) some of the mirrored parts in the very first assembly somehow got moved (even without opening that particular assembly). Subsequently the errors propagated through the whole job as parts where contrained or adaptive to those parts or related parts also moved and the whole model fell over.
It was a couple of days work to isolate and repair the problem parts (by adding additional contraints to reposition them), then finding that more parts in other assemblies then also moved, nightmare.
Now I know about grounding the mirror parts to prevent this (thanks) I will give that a go and hope Autodesk resolve the issue.
I always assumed that when you mirrored a part it created a mirror contraint with the original part.
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