Why doesn't the game allow to assign available officers to assault?

[Aeonarx]

When you choose to assign officers for an assault, there's an arbitrary number of them showing up at the bottom, and some of them do not show up at all despite being available.
The requirement for the position is having "force surrender", and I've got 3 people with negotiation 3 sitting there doing nothing, yet the game doesn't allow me to assign them. What gives?