Unable to Autohint error when exporting

Hello, I’ve been working on a VF for a client and everything has been going well. Recently they asked for the file as two static files rather than a VF. It was exporting well as a VF, but when I got to export static OTFs I get an error message about glyphs unable to autohint one of the corner components.
I added a few extra nodes to it and eventually made it work, but wondering what caused the error in the first place? (my life would be easier without those extra nodes, if they can be avoided!) Thanks

The corner components should not be exported to the final file (neither in static nor in variable fonts). So set them to not exporting.