Polybrush limitations when trying to paint vertex colors?

I'm trying to paint a plane with some special shader that I made that IS compatible with Polybrush, but for some reason it works on some meshes and not on others. On the left, the default Unity plane mesh with the w$$anonymous$$te t$$anonymous$$ngs painted with Polybrush; on the right a mesh I imported from Blender that I can't paint.

Note: the same mesh from blender with less subdivisions is able to be painted, but as soon as I subdivide it, I can't paint anymore.

What can I do?