Similar error: Net Tie Fails DRC - Errors Not Itemized in DRC Dialog
You don’t need a net tie to used different widths per se, but only if you want have different default widths and it’s inconvenient to manually keep two widths in one net. From your text and screenshot I can’t tell if you already know this.
EDIT: here, too: DRC violations / Clearance violation on Bridged Links to filled planes. Maybe this is a bug.