When throwing a projectile underwater such as Snowballs, the projectile will move normally as expected, but later on, it will move backwards and then continue its movement.
This also affects other projectiles such as Eggs, Arrows, but possibly not Tridents. The video below should explain it more clearly, although it might be subtle.
Linked issues
Attachments
Comments 4

Confirmed for 1.13.1-pre1
Confirmed for 1.13.1

fixed somewhere between 1.13.1 and 1.15.2
MC-128812