I noticed the last couple of days while running escort missions that the npc cargo ships that do not belong to the players group show no cargo when scanned when targeted while flying a UDV or hound. I didn't crack any of them open to see if the actually had any candies or not, but either way it looks like a bug to me.

There aren't any NPC cargo ships in an escort mission that aren't part of the player's group. All cargo ships in the escort mission go into the group (even if this pushes the group size far past the eight-slot limit that mere player-created groups have). It's only the escort NPCs (vults, cents, hogs) that don't go into the group.

You were probably scanning the shitty traffic2 bots that serve no purpose other than to make the sector seem busy. They're not part of the convoys at all, and they never have cargo.

Killed a npc trident on Sunday and it was empty.
The other one I killed last Friday was, however, filled with divine spices.