Earlier I linked to a posting by me in another thread.
I should have linked to a posting by me in yet another thread: https://notepad-plus-plus.org/community/topic/77/blank-trim-operations-on-a-text-selection-only
:-D
I guess the answer to the OP’s question is that this (a selection-based action acting only upon the single, most recently made selection [when there are multiple selections active]) has been known behavior for a long time.