All Dolphins around disappear but one in Spectator.

The bug
While I was playing with a friend in a LAN World I used the command /gamemode spectator. I was in an Ocean ruin, with dolphins, but when I used that command all the dolphins disappeared.

How to reproduce
Spawn or find a group of dolphins.
Set your gamemode to spectator by using /gamemode spectator.

All the dolphins will disappear.