The mapReduce documentation currently states for bypassDocumentValidation:

Optional. Enables mapReduce to bypass document validation during the operation. This lets you insert documents that do not meet the validation requirements.

It may be helpful to specify that this option is irrelevant if mapReduce is returning results inline.

One possible edge case: In earlier versions of MongoDB, I believe mapReduce created temporary collections when jsMode was false (default value); this was irrespective of inline mode. Assuming current versions still create these temporary collections, I wonder if there is some edge case where document validation could be enabled on one of these collections while mapReduce was running and cause failures if bypassDocumentValidation was not set (assuming also that bypassDocumentValidation would apply to writes into these temporary collections). This is likely something to ask a server engineer that's more familiar with mapReduce's inner workings.