How to reproduce

1. Summon a new mob with no GUI (strider/pig)
2. Place a saddle on the mob
3. While holding shift, right-click on the mob a few times
4. Hold a warped fungus on a stick/carrot on a stick

The player will be set as the "Passenger" of the mob's data tag when he right-clicked on the mob, but they are not on the mob, which means the mob will have the same values of "Rotation" data tag of the player and each time the player runs a command, it will run at the mob's position (like /summon lighting_bolt)

Also, most of thing that player do, will happen in the mob's position; like if an item is near the mob, the player will pick it up, if the player drops an item, it will drop near the mob, if the player holds a flower, bees will hang around the mob and so on.

This bug persists until the player presses the shift button and they will be teleported near the mob; like they just unmounted or when the mob dies.

This also happens on 20w13a and 20w14~ versions, and earlier versions just on pig.