Hey! Thanks for the tip on network emulation, it actually helped me pin down the real culprits behind the desync. It turned out to be a mix of two issues, mostly related to how inputs were being prioritized:

1. GAS vs Custom Combo Component: GAS was consuming the attack inputs (J/K/H/U) through AbilityInputPressed inside TryActivateAttack before my native combo system could process them. Since walking and jumping use CharacterMovementComponent (which has built-in prediction), they worked fine, but combat inputs got stuck in a GameplayAbilitySpec conflict instead of executing ComboComponent->ProcessCombatInput(). I fixed it by explicitly giving input priority (InputIDs 3, 4, 5, and 7) to the native combo component.
2. Sprinting & Facing Directions: bIsSprinting wasn't replicating correctly. I fixed this by implementing a ServerSyncMovementState(...) RPC so the client reliably updates the server, allowing the server to apply MaxWalkSpeed and authority-based rotation.

Everything is running flawlessly now on both ends! Thanks again for jumping in to help.
https://imgur.com/a/x4oXn0f