Mobs are summoned with their "Air" tag set to 0

Relates to MC-78716

Hmmm... Hard to say if that's really a bug. Depends on whether the summon command also summons a lungful of air along with the entity 😉 When does it update to give them air, and what does a spawn egg set it to?

It updates immediately if they are in air, and it doesn't occur with spawn eggs or breeding. This definitely counts as a bug, mobs should be summoned as they are normally. That's why skeletons can be summoned with armor, zombies can be chicken jockeys, etc.

It's similar to how when using /summon, skeletons don't spawn with a bow in their hand, and mobs never spawn with armor/weapons unless you specify the weapon.

Use NBT in the summon command to provide some air to the mob. e.g. /summon Pig ~3 ~ ~ {Air:300s}