As we know, if a undead(burn under the sunlight) wear a helmet, it won't burn under the sunlight, but this feature is not working for phantoms, even if a phantom wear a helmet(it don't display but it's useful), it will still burn under the sunlight. This issue doesn't happen to zombies, skeletons, strays, drowned or zombie villagers
The command of the helmeted phantom is:

/summon minecraft:phantom ~ ~ ~ {ArmorItems:[{},{},{},{id:"jack_o_lantern",Count:1b}]}