When an entity spawns directly on the ground, the "OnGround" NBT tag isn't set to true for a brief moment. Consequently, entities like parrots and chickens briefly flap their wings upon spawning, and mobs that are immediately damaged after spawning won't experience vertical knockback. To test the knockback issue, you can utilize the command

execute as @p run damage @e[type=!minecraft:player,sort=nearest,limit=1] 1 minecraft:player_attack by @s

in a repeating command block, although it's possible to reproduce the issue without this command. This behavior is related to MC-190551.