Normal WS Blue channel is corrupted after sculpting


I am having a problem with the *Normal WS *node in my material in 4.20.3.
It happens when I use *Vertex Normal WS *or with Pixel Normal WS.

I have made my landscape using a heightmap (16bit PNG, if that matters). There it worked just fine.
Then a few sculpt strokes were made to optimize the landscape for the game and it lost the information of the blue-channel. It became 100% white, where I have used the sculpt tool.
Even the slightest brush stroke turned the blue channel to white for that vertices.
Only the blue channel is affected. the other channels seem to be working just fine.

When I rebuild the material from scratch, it’s just the same.

I hope, someone has an idea.
Kind regards