Failed to start mongo.service on ubuntu 16.04 after update from 4.0 to 4.2
Interesting.. It should not be possible...  From the source:
    cpp_name: oldOplogDeleteFromPoint
￼                type: timestamp
￼                optional: true # This field only exists on 3.4 upgrade
￼                description: &quot;The timestamp of the first oplog entry in a batch when we are writing
￼                              oplog entries to the oplog after which the oplog may be inconsistent.
￼                              This field only exists on 3.4 upgrade.&quot;

And this problem only exists on unclean upgrade from &lt;=3.4 version. It is removed from source code at version 3.8 forward.
Yes, I know, this answer doesn't solve the problem, but... It looks like that your upgrade process was not &quot;by the book&quot;, sorry.