It was actually a bug that it used the preview weights on a blank landscape, and this is no longer the case in 4.13 onwards. The preview weights were only intended for the material editor preview.
The correct solution is to right-click one of your layers in the landscape editor and choose “Fill”. This will fill the landscape with that layer, and then you can paint normally from there.
Also, if you assign your material and layers before creating your landscape, the editor will automatically fill the first weight-blended layer for you.
Let me know if you have further questions or need additional assistance.
Cheers,