Unreliable Raycast on Terrain

Firing a grid of raycasts onto terrain used to give expected results, now it fails in a small percentage of attempts. Here is my RaycastParams:

local rayP = RaycastParams.new()
rayP.IgnoreWater = true
rayP.FilterType = Enum.RaycastFilterType.Whitelist
rayP.FilterDescendantsInstances = {workspace.Terrain}

Here is a video where I display missed ray casts which return nil using beams:

The cases near the ocean are to be expected, but the ones in the middle of the island are scattered and generally without a consistent global pattern I can discern. (edit: accidentally recorded the song playing at the time)

Because I had no trouble before, this sounds like an engine bug involving terrain collision detection. However, I can’t post there, so could someone move me there or suggest what might be going wrong on my end?