I had a similar issue to this, not sure if it’s exactly the same, but I wanted to document it here in case others run across it as the solutions here didn’t fix it for me.
See below for a video of my issue. Notice the cube that shimmers, based on screen size / viewing angle. This only happens to that cube because it’s the only one I’ve moved, but after moving it once, it’s stationary. Also in my case I’m not using HISM but just ISM. Turns out the fix was to update its position using UInstancedStaticMeshComponent::BatchUpdateInstancesTransforms instead of UInstancedStaticMeshComponent::UpdateInstances. In my case I was using ISMs via the Mass Entity system, which by default uses the latter. Thankfully, there’s a console command to switch it to the former, via Mass.CallUpdateInstances 0. I still don’t know why UpdateInstances causes this, seems like a UE bug.