Ah, so you doing it good calculating the speed in the animBP! But as far as I see in:
https://github.com/EpicGames/UnrealEngine/blob/bf95c2cbc703123e08ab54e3ceccdd47e48d224a/Engine/Source/Runtime/Engine/Classes/GameFramework/CharacterMovementComponent.h#L253
The maxWalk speed you are changing is NOT a replicated Property. Sou you should change that in a Multicast which will change that value on server and clients, or use your Bool from the Pawn inside of you animBP Logic (sounds like multiplying the speed when bool is true - but doesn’t sound “clean” for me).
Here is a tested code - you could skip the SwitchHasAuthorit and call always the serverSet - it doesn’t make a difference - but I feel it is cleaner that way: