The bug
As shown in the image phantoms going up or down in a minecart take suffocation damage.
Reproduction steps
Build a rail slope as shown below
[media]Summon a phantom in a minecart
/summon minecart ~ ~ ~ {Passengers:[{id:"phantom"}]}
Push the minecart up the slope
Linked issues
Attachments
Comments 8


Still in 20w11a

In 20w12a

Affects 20w45a
Affects 1.16.4
Can confirm in 21w05a. Feel free to use the following command to make reproducing this much easier:
/summon minecart ~ ~ ~ {Passengers:[{id:"phantom"}]}
Can confirm in 21w06a.
MC-38072 and it's resolution may be relevant to this issue. I can confirm in 1.20.5 Release Candidate 1.
Fixed with the 24w34a minecart experiment:
[media]
Would not really call that a bug, as the phantoms hitbox is in the stone when the minecart goes over it. Just like players that suffocate when going through stones above a minecart track.
That here seems to work as implemented...(Even if that might be an inconvenience in survival if you try to move phantoms like that)