All minecraft projectiles behave unpredictably when attempting to copy one entity's motion to them through the /execute store command.

How to recreate:

-Place down three command blocks with one of these commands in them:

/execute store result entity @e[type=potion,limit=1] Motion[0] double 1 run data get entity @e[type=player,limit=1] Motion[0] 1

/execute store result entity @e[type=potion,limit=1] Motion[1] double 1 run data get entity @e[type=player,limit=1] Motion[1] 1

/execute store result entity @e[type=potion,limit=1] Motion[2] double 1 run data get entity @e[type=player,limit=1] Motion[2] 1

-Throw a potion. Instead of following the player's movement, the potion simply acts as though it collided with an entity or a block. This does not change when firing it out of a dispenser.

Similarly copying a snowball's motion on to a potion makes the potion fly off in a completely different direction until the snowball starts descending, where it breaks on the ground.

This can also be recreated with arrows, snowballs, eggs and xp bottles, so I'm under the impression this bug affects any projectile affected by gravity.