When tweakEmptyShulkerBoxesStack is enabled, removing the dye of many coloured Shulker boxes at once gives back only one un-dyed Shulker box.

Because the vanilla code that handles washing the Shulker Boxes doesn't check for the stack size, it just assumes that it's going to always be 1. Which means that I just need to add an additional fix for that code, which is sort of entirely unrelated to the Shulker Box stacking tweak itself.

By that I mean that it's not that the stacking tweak itself should be responsible going all over the game code and changing stuff in a million places to fix stuff. It should (and does) only allow increasing the stack size for those items. Shulker Box related code elsewhere is just hard coded to assume a stack size of 1 and it doesn't even check the current actual stack size when doing things.

Why do you say that this is a vanilla bug?

Ok thanks for clarifying.