Copy Subtasks does not copy subtasks into their current project
When copying issues which have subtasks in other (sub) projects not the same as master issue, the subtask issues are copied into the project of the master issue, instead of the project they are currently inside.
Horse |- Foal |- Pony
Horse - #1 Hooves Foal |- #2 Tail Pony |- #3 Fetlock
#1 - Hooves with subtasks and you get
Horse #4 Copy of Hooves Horse |- #5 Tail Horse |- #6 Fetlock
Horse #4 Copy of Hooves Foal |- #5 Tail Pony |- #6 Fetlock
(in addition, the copied subtasks have identical titles, but that's another issue)