When you have a dolphin in a block of water with air above it, when a player nearby is in creative, survival, or adventure mode, the dolphin will grab air every now and then.

However, if everyone in the local area is in spectator mode, it does not go to get air and then it eventually dies.

You can check this by putting a dolphin into the block of water and waiting in creative mode using the "/data get entity @e[type=minecraft:dolphin,sort=nearest,limit=1] Air" command and you will see it replenishes.

Then go into spectator mode and do the same command and you will see it will only go downwards until it dies.

This is a recreation of an issue reported on a modified client, tested, and continues to be a bug on a vanilla client.