Trees not spawning on custom grass block in custom biome

As the title says, the custom trees created (Not using schematics, using the generator in the "Basic Properties" section of the Biome maker) will not spawn on the custom block. They only spawn on regular grass blocks just outside of the biome. Currently using the latest 2021.1 snapshot as of this post's date (b03117) so it could just be a bug, but if there's a reason for this other than buggy snapshot I'd appreciate some help.