When holding a brush and facing a block without collision, the hitbox of the block without collision is visible rather than the one you will actually target if you use the brush

You can brush blocks through blocks without collision like torches and tall grass, but when you face them while holding a brush, the hitbox of the grass/torch/other block shows instead of the block behind it.

This is kind of a gray area of whether or not this is a bug being that if you "attack" it, it will still attack the block in front, so I wouldn't be surprised if it turns out this is intentional.