Basically the title. I noticed there was another issue open that you closed. My config is completely default. Here are some screenshots:

Showing it floating attached to an island:

Showing it embedded in an island:

Showing it floating in the middle of nowhere:

Requesting a copy of your config.

@Vazkii It's the default one. Just tried on a clean install of Forge and Quark as well.
quark-common.toml.txt

Turns out this was caused by a typo in the allowed values for the config not allowing the end biome category to be in the list (it's the only biome in which the code name and logical name differ). Since the big dungeon default spawn config is a blacklist, it failed to recognize the end entry, and would let them spawn there.

This will be fixed in the latest update, but you'll have to regenerate that part of the config yourself to see the changes.