On a dedicated server with allow-flight=false, players with a generic.gravity attribute of 0.0 are kicked the instant they are not on the ground.
To reproduce:
Run a dedicated server with
allow-flight=falseRun
/attribute @s minecraft:generic.gravity base set 0.0Float to a location where the block below you is air, or mine the block below you
From the Minecraft Wiki:
minecraft:generic-gravity Information: The gravity affecting an entity in blocks per tick squared. Default Value: 0.08
server.properties: allow-flight: Whether players can use flight on the server while in Survival mode by using mods.
With allow-flight enabled, griefers may become more common, because it makes their work easier. In Creative mode, this has no effect.
false - Flight is not allowed (players in air for at least 5 seconds get kicked).
true - Flight is allowed, and used if the player has a fly mod installed.
The server doesn't know if any attributes are affecting the player in any way, therefore causing the server to kick the player.