Dolphin's are supposed to give the "Dolphin's Grace" effect to players swimming nearby. However, they don't do this if the player has the "Blindness" effect.

I think this is a leftover bug. Dolphins are supposed to grace blind players but not invisible ones. In 1.14.4 this was the wrong way around - invisible players were graced but blind ones weren't.
In 1.15.1 this bug was partially fixed - now invisible players don't get graced any more but blindness still prevents Dolphin's Grace.

Reproduce:

• Spawn a Dolphin (best trap it in a contained area or you might not find it when blind)
• Go to survival mode
• Drink an invisibility potion and/or eat an Azure Bluet Suspicious Stew
• Swim near the dolphin
• Observe status effect icons