This wasted a lot of time for me, so hopefully it prevents the same from happening to others. If you have a timeline that’s supposed to replicate, setting the blueprint actor to “Hidden From Game” will disable that replication.
This easy to test - just have a blueprint actor print string the value of the timeline output and start the timeline with delay if it’s not the server. Set the timeline to Replicate, and run a PIE test with two players (one is the listen server). This will print values in sync.
Rerun the test with the actor “Hidden in Game” checked. The values between server and client will then be out of sync.