You may have found a wee bug. This is a function I rarely use but the GG does say
| J | Rejoins gun or vehicle crews unloaded from transport aircraft during deployment |
But pressing J during deployment opens the unit roster which is not helpful to you in this case. ( FYI that was changed to calling up the roster from rejoining in 2013 but updating the GG was overlooked......I **think** there was some weirdness caused in rejoining but I'm confering with Andy about that )
That said if you load a mortar like that into a transport you will notice that it breaks the unit down crew/ mortar.......that is intentional, both drop separately and once on the ground the crew needs to be re-unitied with the mortar so they won't rejoin until you re-unite them in the same hex for a turn . So, in short.....that's the way it's supposed to work so mortars aren't dropped and ready to fire right after landing ...but the J shortcut useage needs to be corrected in the GG