If you attempt to generate a superflat, the game functions as intended. However, as soon as you enter the superflat customize menu, the option will bug and instead select garden of glass, as though it had been re-initialized. This has the ultimate effect of erasing all superflat customizations, since returning to the More World Options... menu effectively clears it by resetting to GOG. Cycling back to superflat manually, as expected, institutes default values.

This is most likely caused by the default world type mod used to set the garden of glass world type as default. I noticed it myself when I was first making the pack, but never really looked into it as I didn't consider it that big of an issue.

For now, if you need a customised superflat world you should be able to just remove the default world type mod and it should work as expected. I'll have a better look into it at some later point and see what I can do for a permanent fix.