Sorry to get back to this so slowly.
I am no longer able to reproduce. As of 425preview1 I was, but no longer with this issue in the 425preview2; the landscape material imports, compiles, and performs as expected from 423.1.
There is a new issue where just loading the landscape into the level makes ~10meg accrue every second, even idle, running me out of memory eventually.
I’ll wait until the full 425 production release to test and log a defect therein if the memory leak presents, but otherwise the issue w/many-many shaders is no longer extant…