There is a difference in classic and SBE when running $_resumeAfter with clustered collections where SBE can crash the server (steps to reproduce are below). This will affect all versions that use SBE for clustered collections (right now only 7.1)

I think this is partially because how SBE and classic handle null input to $_resumeAfter. Classic and SBE return different results for the postBatchResumeToken after all the documents are returned in the collection. When we have no more documents left, classic returns {$recordId: null} and then after returns the first document recordId. SBE just returns the last document recordId again and again. SBE does not return a null value.

Either both engines should return the same output, or at least SBE should not crash when run with {'$recordId': null}. This was discovered in another bug investigation (SERVER-77386).