No matter the value, and even if the tag does not exist (such as "/summon minecraft:dolphin ~ ~ ~ {TagThatDoesNotExist:1}"), dolphins spawned with a tag will have their "Air" tag set to 0. So they will instantanously start to take damage and try to reach for the surface. So if they can't (or can't do that in time), they will die really quickly.

If you add "Air:4800" to the command ("/summon minecraft:dolphin ~ ~ ~ {TagThatDoesNotExist:1,Air:4800}"), it will fix the problem if the dolphins are in water. But if spawned on the ground, they will still instantanously start to take damage (whereas dolphins spawned on the ground without ANY tag (not even "Air") will need some time before taking damage. Same if they are spawned in water (with a tag) and then teleported on the ground. In which case they might take damage from drowning before being teleported, but that's all).

I've been unable to find (with /data) what tag counts how much time dolphins can stay out of water (it seems to be hidden, but it's probably not supposed to), but it must be set to 0 as well.