From the Krita Docs on the Outline Selection Tool (lasso):
Holding Ctrl key while drawing the selection temporarily makes this tool to behave like polygon selection tool and you can then draw straight line selections by just clicking on canvas.
The problem that I’m finding is that it seems Ctrl also makes the Outline Selection Tool go into Replace mode, so you can’t Add to or Subtract from your selection while also using the Ctrl key to select in straight lines.
I tried to find a way to unbind Ctrl from Replace mode selection, but I think this is hard-coded? I’m already using Shift and Alt to choose whether I want to add or subtract, and personally I don’t see a reason to have Ctrl bound to Replace mode at all. Anyway, it does seem to conflict with the straight edge selection mode that is also assigned to Ctrl so this seems like a bug or a bad design choice.