The txnBytesDirty metric gets reset whenever a query yields. The consequence of this is that a long-running operation, despite updating many documents and rewriting many index keys, can have an artificially low value for txnBytesDirty. The ask here would be to persist this metric across yields.

Edit: Following this comment and this comment in the codebase, it appears that this value is reset whenever a transaction is committed or aborted, with yielding being just one example of a way that a transaction can be completed. So the broader issue is that this value can be reported a zero for an operation that did perform writes.