I had this exact same issue. Believe it or not, it has nothing to do with replication or movement settings. The problem was that there was a collision occurring between two or more components in the pawn. It doesn’t cause any problems until an authoritative server is added.
For example, your gun could be colliding with your capsule…
To find the issue, I simply started removing components until the lag stopped. When I found the culprit, I messed with the settings until I found the fix. Good luck!