I believe the ‘lost’ parameters only get lost when they reside inside a material function. your repro doesn’t attempt to do that - I know the original repro steps didn’t mention it but they do now, as well as the comments below
given the seriousness of the issue, if that still fails I’d suggest the following:
- from the marketplace import any of the “Realistic Landscapes Bundle” packs into a new project
- open any of the showcase maps
- notice the material instance parameters are lost
- repeat in 4.18, and see how everything works correctly