I’m going to answer this in case someone else has this problem. There was a box brush area below the landscape that was about 2X larger than it should have been, extending beyond the mesh it was intended to be under, placing it in close proximity with the landscape at one end. When I shortened the dimensions of the mesh to fit, the landscape quit misbehaving, and I haven’t seen it err since.
This may not happen in 4.20, but it sure messed up in 4.19.