This also happens with vertical warps, but since the player is forced to play an especific frame when warping it's harder to notice. However:
2.) If a warp has a scroll effect and the section it's enough big, you will see an odd movement for the camera.
(Notice how in this gif the camera doesn't go into a straight line)
This happens because the camera follows its way to the warp's position, but as soon as the player is getting out the pipe, the camera aggressively focus on it causing these "camera gaps".
To fix this, there should be a calculation to know the exact position where the player is will be leaving the warp. (For example, an exit up warp should get the warp position, substract it 32 and the player height.)
3.) The second player plays the sound effect "warp"/"shrink" when entering from a door. The game should check if he's entering from a pipe warp or for a door and play the corresponding sound effect.
4.) Not a bug but a silly suggestion. Can you please make a warp switch into "Exit" after placed the "Entrance" warp?