I’ve encountered the same issue and investigated its root cause. Groom components rely on a Niagara component for simulation under the hood, and all groom assets are evaluated together in a single batch. This simulation runs on the GPU, which explains why adjusting the tick order does not resolve the problem—the simulation update occurs on the render/RHI thread rather than the game thread.
Because the simulation is integrated into the main scene rendering pass, disabling the main viewport rendering also disables the groom simulation.
Here is the frame capture (DUMPGPU) of a frame in my game
I didn’t solve the issue yet but I’m ensure that this requires some engine adjustments. Even if so I write down the solution as soon as I find it.