details:
https://discord.com/channels/882822986795716608/894946828980723762/1040993163260731392

box item entities can merge wrongly. they only need to be "empty" and has any tag to be merge.

if i rename empty boxes. then a box named "hi" can merged with a box named "hello".

and box with loottable is viewed as empty too.(they should not)

so,if i give myself 2 box of spider and zombie:
/give @A minecraft:shulker_box{BlockEntityTag: {LootTable: "minecraft:entities/zombie"}}
/give @A minecraft:shulker_box{BlockEntityTag: {LootTable: "minecraft:entities/spider"}}

those 2 boxes (and hi and hello) can merge with each others.

Cannot repro different color merging

code that only check if they both have tag:
https://github.com/gnembon/fabric-carpet/blob/master/src/main/java/carpet/mixins/ItemEntityMixin.java#L94
code that forget loottable:
https://github.com/gnembon/fabric-carpet/blob/master/src/main/java/carpet/helpers/InventoryHelper.java#L59

update: @x1t0m1t0m1x found that shulker box s color is ignored as well.

https://discord.com/channels/882822986795716608/1042722371158683648/1042722371158683648