The changes from SERVER-50778 (5.0) made it so $set applies numeric paths in numerical sort order. In 4.4, $set applies numeric paths in lexicographical sort order. https://www.mongodb.com/docs/manual/release-notes/5.0-compatibility/#update-operator-processing-order

The changes from SERVER-50778 were neither guarded on the 5.0 primary node by its feature compatibility version, nor were the changes to how the oplog entry instructions ought to be interpreted by secondaries made explicit in the oplog entry itself. This ambiguity allows a 4.4 secondary node to receive the oplog entry instructions and apply them differently than the 5.0 primary node had originally done. The reverse is also true; a 5.0 secondary node can receive the oplog entry instructions from a 4.4 primary node and apply them differently than the 4.4 primary node had originally done.