I'm apparently not the only one getting this exact same issue with Vega 56 playing Sekiro with the latest patch. Every fivish seconds the GPU clock frequency briefly falls off a cliff, dragging the FPS down with it. I play on 1440p and it doesn't matter if I play with Max or Low quality, FPS is capped at a solid 60 for a majority of the time, but randomly dips to below 30 in all environments and situations, even when nothing is happening and the player character is not moving.
My i5-3750k is not approaching max load on any of the cores at any time, so it's not a CPU bottleneck either. It also seems like this particular issue is unique to Vega and does not affect Polaris cards as far as I have been able to tell.