I commented on a similar topic here.
I don’t use ACAD. I use PowerCADD and it doesn’t use polylines to make thick lines. I have seen that in ACAD files from others, like surveyors. I think corruption creeps in by means it’s often hard to figure out. Accidentally nudging something? It is utterly frustrating when it happens. As I mentioned in the other post, I keep a trail of saved files to go back to and retrieve unspoiled geometry from, but even that has proved tricky.