Weight maps - let’s start by call them what they are hu?
Layer paint layers, or splat maps -
They do not need to be png16.
In fact, thats probably the main issue. The files you are importing end up compressed and altered by the engine into values that do not fully make sense.
Save them as regular old PNG at whatever the regular stuff is bit wise (hint, its 8bit but shpuld just be named PNG without any number in all programs).
Weight maps arent really a thing.
The other issue:
If you are using height blend in the material, you need height blended sample layers when you create them.
Or you shpuld also be able to open the asset and upadate the checkbox for that now a days (used to be you had to delete/re-create).
Aside from that. I have never seen the artifact you are getting before, so I’m also sure there is something incorrect going on with your material. No idea what ofc…
Try the file change and layer thing first. Update here if it persists.
As another aside:
Any landscape material with more than 3 paint layers that are textured is automatically just wrong/against best pracrice.