Or, I have a mod modifying brick stack sizes without knowing it, and without saying so on the mod page. Whoops.
No clue which mod does so, but vanilla doesn't allow 16 bricks to be fired, only 12. I thought it had changed in 1.21.
Edit: It appears to be Bricklayers doing the shift from 12 to 16 bricks, so perhaps the bug starts there.