I had the same problem. As it just happened to coincide with Civil 3d update, I save the DWG in 2026 and it seemed to clear up the problem. It has not happened since so I do t n owing the civil 3d update did something with tbc but idk
I have the same problem. I used the cleanup utility, installed 2024.12 and that didnt fix it. checked my windows updates and that didnt fix it. It is super frustrating because almost every model I create relies on imports from cad in one form or another including dgn files.