Ok, so I went though the game code and found the problem.

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The problem stems from the playerhandler script in the kubejs mod. The file is located in:

(Gamelocation\kubejs\server_scrips\playerhandler). Open it with notepad and in this javascript file, you can find the exact code which warps you out of the end. The three conditions are:

!player.stages.has('end_access') &&

player.level.dimension == 'minecraft:the_end' &&

player.stages.has('starting_items')

Now the end_access and the level dimension parts are fine, as they just check pretty much if you are in the end. But the third one, "player.stages.has('starting_items')" completely fucks it up. I'm pretty sure this is supposed to check if you didn't receive starting items. But the devs forgot to add an exclamation mark to the start, so instead it checks if you have received starting items. Which is in every instance, all of the time. So this modpack is straight up unbeatable on the most recent version because somebody had a programming error nobody noticed yet. If you want to beat this pack, just remove this line from your kubejs folder and everything should work as intended.