Logically, it would seem it is caused by the distance field on the rocks.
Ergo, I’d just assume you did something incorrect within the material - or that there is yet another material node/rendering bug.
Distance to nearest surface does use the distance fields.
You can attempt to visualize the rocks using something like DistanceFieldGradient and a high power value to increase the color range.
Remeber that landscape and whatever else also generates the diatance field required to visualize within the material
And that a water sheet should not generate distance fields itself.
The artifact is likely caused by whatever is within the water.
Definitely should not flicker, but there could be 1000 reasons why it does.