This actually isn’t a bug, instead both materials for Parts and Terrain are grouped together under the same enumeration. There is an upcoming release that should give the ability to use terrain materials for parts, but as far as I am aware that is a separate release build and isn’t an actual Beta in the regular Studio yet.
The system is already ready for this update but the materials are not yet in the files so we could receive an announcement in the next 2-3 months with the release date of the materials
I agree that lumping the materials all together under one Enum isn’t a bug, but having the Material properties on the Terrain and BasePart pages in the API both link to the same BasePart/Material page (which in turn links to the Enum/Material page) without explaining that materials can be exclusive to either terrain or parts is confusing. Maybe it’s preferable to have all the mats in one enum for future growth or whatever, but it ought to be easy enough to clarify which Materials are for what on the individual reference pages (would prob need to be noted on at least the BasePart/Material and Enum/Material pages).