If you give a unit a CSLA PRC-77 backpack in the Editor Arsenal, copy and pasting that unit will cause a "bad vehicle type" error, and the pasted unit will not have a backpack.
Preconfigured units with a PRC-77 (e.g. US Airborne RTO) can be safely copied, until you edit their loadout. Saving the loadout in the Editor Arsenal, even if no actual changes are made, causes the "bad vehicle type" behaviour to start occurring.
Units saved with PRC-77s as custom Editor compositions also exhibit this behaviour.