I had a similar imploding model issue, which put simply was that there were too many segments in one loop. Drove me nuts until he inimitable Box twigged to it and then sWilliams explained the technical realities: “We have a hard limit on the number of edges in the algorithm that detects new loops. If the algorithm traverses 10,000 connected edges without hitting a repeat, it gives up. I believe that the workaround (create several faces and then merge them) uses another code path.” Here’s the thread: Instant crash when raw geometry of dwg import is manipulated