When any aquatic mob that flops, which is cod, salmon, tropical fish, pufferfish, dolphin, and tadpole, is killed out of water, they continue to flop during their death animation. You would expect them to stop flopping as it doesn't make sense for them to continue to do this as they are dead. Another thing about this is that the guardian and elder guardian stop flopping when they are killed on land so I would think this is meant to happen.

To replicate this, use one of the following commands to summon one of the mobs I have listed above and see that they continue to flop around even though they shouldn't be.

/summon cod ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon salmon ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon tropical_fish ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon pufferfish ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon dolphin ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon tadpole ~ ~ ~ {Health:0b,DeathTime:-200s}

You will see that the listed mobs above continue to flop around even though they shouldn't be and you can compare this behavior to the guardian or elder guardian in the two commands listed below.

/summon guardian ~ ~ ~ {Health:0b,DeathTime:-200s}

/summon elder_guardian ~ ~ ~ {Health:0b,DeathTime:-200s}