I've seen this bug on the Steam forums but figured I'd put it here as well.
When a unit is disbanded, there is no guarantee it will come back. By that I mean, after an operation, replacements are brought in, and the unit may show up in the "soldier" screen but will not be made available to select for battle.
I purposefully got some units destroyed/disbanded to test this out:
On the "First Blood" mission, I got my 3rd Squad Rifle team destroyed and let my 1st Squad BAR team lose all but 1 member and they broke. Both units were disbanded. I played the 2nd day of the "First Blood" mission and saw that the two units had returned (The rifle team with 6 new people and the BAR team with the one survivor and 5 new guys).
I was able to see these replenished units in the "soldier" screen but wasn't able to select them as part of the main force. I thought that maybe the issue was that I was still in the middle of an operation, and they'd come back when I started Sbiba...but they did not. I still see them in the soldier screen, but can't select them for my main force.
I haven't verified this yet, but I'm not sure this happens EVERY time a unit is disbanded. I swear I've seen some disbanded units come back.
Regardless, this needs to be fixed. As these units slowly become unavailable due to the bug, I'll eventually only have like 50 guys left to start a mission that requires a 150 person company.
Thanks in advance!