If you have an item entity between you and a block you are trying to brush, the game won't let you brush it. However, the vast majority of other both left and right click actions are still allowed through items, which makes me quite sure this is not intended.
Steps to reproduce
Place suspicious gravel
Throw random stuff on the ground in front of the suspicious gravel
Fill up your inventory with brushes so you don't pick up the items
Go in survival mode (again so you don't pick up the stuff)
Try to brush the suspicious gravel through the dropped items (it actually doesn't need to be a brushable block, but suspicious gravel is a good indicator for this example)
Observed result
❌ it won't let you because the item is in the way.
Expected result
✔ the item being in the way does not interfere with your ability to brush blocks
6. Hit the block
✔ it lets you hit the block through the item, which means items are ignored in most operations
Interestingly, the area it doesn't let you brush is a lot bigger than the actual size of the item's hitbox.
This is most likely caused by the fix of MC-260898.
Can confirm, and i want to add that this isn't exclusive to item entities, because it also happens with area effect clouds, experience orbs, and most likely other similar entities that don't block player's actions through their hitbox.