Currently, the RollbackFuzzer calls dumpCollectionDiff() when it detects a mismatch in collection stats for any of the fields "capped", "nindexes", or "ns". It does not, however, tell you which of the fields actually had a mismatch, and the dumpCollectionDiff() function only shows collection document differences anyway.
I don't think it's helpful to call dumpCollectionDiff() for this failure case. Instead, it should just display the output of collStats for each node, and also display the actual fields that triggered the problem. The output of collStats is enormous and is output to a single line, making a determination of the failure reason unusually difficult.
You can see an example of this in https://logkeeper.mongodb.org/build/222903c574e9dc03a763ab16015f31f8/test/5d1a64fec2ab684c9109a185?html=1#L20988