The bug
When using /data modify to append the entries of a list into another list, they get appended in reverse order.
How to reproduce
/setblock ~ ~ ~ minecraft:chest{Items:[{id:"minecraft:stone",Count:1b,tag:{list1:[1,2,3],list2:[1,2,3]}}]}
/data modify block ~ ~ ~ Items[0].tag.list1 append from block ~ ~ ~ Items[0].tag.list2[]
/data get block ~ ~ ~ Items[0].tag
This results in the list [1,2,3,3,2,1] rather than the expected list [1,2,3,1,2,3]
As a side effect, "insert after" and "insert before" are merged into "insert". When inserting, index 0 means "before first element", while -1 means "after last element".