Can confirm it's not the printer, just set up a new CR10s Pro V2 yesterday and it's doing the same thing. I've uninstalled/reinstalled Cura from my computer and that didn't fix it either. It's not really ruining any of the prints, and before this happened I wasn't really sure what gcode even was, so on the bright side it's forcing me to learn stuff 🙂
I might try rolling back to a previous version, but for now I'm just removing the triangle as soon as it's printed if I'm quick enough.