When wearing boots with Depth Strider III and receiving Dolphin's Grace, all fish and dolphins despawn from loaded chunks when Dolphin's Grace ends.

To recreate (best with two players):

Spawn a bunch of dolphins and tropicla fish in an area:

/summon minecraft:tropical_fish ~ ~ ~
/summon minecraft:dolphin ~ ~ ~

Have the first player wait near the fish and dolphins, and have the second player that is wearing the boots with Depth Strider enter "swimming" mode (sprint key).  One or more of the dolphins will follow them giving them the Dolphin's Grace effect.  Keep swimming without running into anything, and when the Dolphin's Grace effect ends, all the fish and dolphins will despawn.