The "Upgrade all" button can upgrade bandits into normal troops without the disciplinarian perk. It should do a check before doing so and ignore those troops if you don't have it.
Additionally, this is not a bug report but a suggestion. When upgrade rules are finally implemented, "don't upgrade bandits" should be implemented as a rule for role-playing purposes.
The "Upgrade all" button can upgrade bandits into normal troops without the disciplinarian perk. It should do a check before doing so and ignore those troops if you don't have it.
Additionally, this is not a bug report but a suggestion. When upgrade rules are finally implemented, "don't upgrade bandits" should be implemented as a rule for role-playing purposes.