@howdev its interesting to get differing perspectives, all I would note is that neither is "right", they are just different, but we have sidetracked this issue enough.
Back to your OP, does the bbb line have a line-end on it? I suspect not, so the paste at the start of line aaa does not separate the lines. If the Vim `p` command is to paste whatever is in the buffer as lines, not as text, then the vimode plugin is going to have to modify the clipboard if its missing a line end because the normal paste in Geany is to paste text.