I tried your approach with a new project and it worked fine with landscape and material - after a successful recompile of the master material. It made me think that recompiling the master material would solve something - and it did. The landscape had some “geometry” issues corrected from 4.12 to 4.13 (I was prompted to correct them), I guess it could have had something to do with that?
For now I consider the issue resolved. I can still provide the callstack if necessary.