So, it’s pretty impossible to determine if this is a bug without seeing the log, the stats of the brother in question, and more detail around the sequence of events in the battle. A couple reasons you might have seen the chance you did, though:
– Elevation affects chance to-hit: if the auxiliary was above your brother, that’s a flat -10% chance to hit
– Without Flail Mastery, flails ignore the shield defense bonus but not the Shieldwall bonus. If the Auxiliary in question had Shieldwall active, you would have seen a decrease in chance to hit.
– The number of friendly brothers immediately adjacent to a target increases a brother’s chance to hit (a flat +5% per adjacent brother). That might have also affected your chance.
– Lastly, although I don’t think it was at play here, but Morale affects (among other stats) Melee skill – if your brother goes down to wavering or breaking morale, he will see a lower chance to hit (due to his lower melee skill). Again, from the log it looks like there wasn’t a morale chance that affected things.