1.16.4-1.13.9
Title. Having both the attribute "% more projectile speed" and "% more projectile damage" on a bow causes arrows shot by that bow to desync by ~10° to the left or right for about 1 second after firing.
https://streamable.com/qhpf0q

Still in 1.16.5-1.13.12, but it appears I was incorrect about the cause. It seems to occur only when projectile speed goes over +200% or so, from perks and bow attributes.