In resource packs, a blockstate file may be formatted to begin with the following:
{ "variants": { "randomised": [ etc.
in order to randomise the choice of model used when rendering a block. However, in 1.20.2, blockstate files using this randomised variant functionality no longer work, despite the same files operating correctly in 1.20.1.
I believe this is a bug, as I could not find any reference to a change in format to randomised variant models in the changelogs, but correct me if I'm wrong.
Check the vanilla grass block block state file, or stone, or sand. It still works, you just need to format it properly.