Player#getLocation() returns a wrong location where Location#getY() is always the ground level when the player is riding a pig, a horse, a boat, etc.

The player's NBT Pos data shows the correct position the player should be in, and there can observe a vibration of NBT Pos Y flipping from the true value and the wrong value from the spigot.

Example: a player riding on a boat on ground level 68.0;

getLocation().getY() print:

Player NBT Pos data flips:

Example: a player riding on a horse on ground level 64.0;

getLocation().getY() print:

Player NBT Pos data flips:

There is an exception that it returns the right value when the player is riding on a minecart:

In the vanilla game without a spigot, there is no NBT data fluctuation, as shown below: