Datapack Trouble - works only in newly created worlds (1.21.1 Java)

I have a problem with a datapack I updated to 1.21.1 to use it on my server. In a newly created singleplayer world everything works just fine. When I load it onto my modded server, reload, disable and enable it, it does not work. I can use /function to trigger the function and it works as intended, but it wont work automatically which is bad for a auto pickup datapack. After trying a few things I found out that the datapack only works on newly created worlds, not on existing. Do you have any idea? Log: https://mclo.gs/H4cAQB4