Hi,
This issue sounds unusual, but I wouldn’t recommend running mixed major releases for an extended period of time beyond validating your upgrade.
It has been some time since you posted this question - were you able to find a solution? If you are still running mixed versions, what specific version(s) of MongoDB 3.4 are you using?
One possibility is that the collection metadata (count & size stats) is somehow incorrect. Usually this happens after an unexpected shutdown, but you could try to run validate on affected collections to see if this corrects your issue.
Regards,
Stennie