the bonus player ability could be the repeatable research at the end of the warfare tree (+10% each time you research it). the unit seems to be fortress built (displays 35 defense - master chain is 26 def, ironskin adds 3, the remaining 6 are probably from an armorer building), so that would explain some of the extra traits (impulsive/enmity from foretress level & warrior temple [kingdom version]) and i think AI units get some additional trait slots or bonus traits at higher levels.

so i think there's no bug involved.