If you right-click on a BrushableBlock and hold the click down as the mouse moves from one BrushableBlock to another, if the second block instance has no held items...neither the PlayerInteractEvent nor the BlockDropItemEvent fires. Thus, the PlayerInteractEvent doesn't fire on the interaction with the second block.

Repeating the issue:

Click on the first BrushableBlock and start the brush animation, which fires the interact event. Using the same continuous animation as you move across different blocks doesn't register any additional interaction events, despite the brushing animation executing against additional BrushableBlocks.