Code doesn't really address this, but on occasions where previous installers have not left the required 1/4" sheathing in the box, I've "fixed" it by installing a piece of heat-shrink over the conductors to replace the missing sheathing. The sheathing is not insulation, but there to hold the individual conductors together and protect them during installation and where they enter through fittings/clamps. The heat shrink provides similar protection to the missing sheathing. I've never had an inspector (they are the AHJ) have an issue with this. As long as the wires were not cut exactly to the 6" limit, this is an option.