You can try replaying the fight on v0.43+ or v0.42-, but if that doesn't help then this is something only the dev can fix.
Information the dev might need to figure out how this happened:
1. Which version are you on?
2. Did you initiate the Warlord fight through Bradley, or Arthur?
3. Where was Bradley during the fight (dead, imprisoned, or with you at the battle)?
4. Where were Rylan and Riordan (dead, imprisoned, chained to your bed, forbidden from participating, or with you at the battle)?
5. Did the combat have a frontline and backline? If so, which characters did you put in which lines?
6. Which characters (if any) died during the battle?
Explanation:
You get that message only after visiting Riordan's house for the first time post-Warlord IF you previously agreed to help him end his existence by taking him to the Battle against the Warlord (setting "riordan_askforhelp" to 2). So he should have been added to "arihama_soldiers" and thus been at that battle for you.
But (at least in the latest version), if he died in the fight, riordan_askforhelp should have been set to 5; alternatively, if you defeated the Warlord and riordan was in "warlord_party", it should have been set to 6. So there should be no way to make it out of that battle with riordan_askforhelp still set to 2 if Riordan is there - hence the dev wanting you to report that.
For the dev:
It would seem that you may not have properly considered Riordan's flags when porting Arihama to the new combat system - there doesn't seem to be any way for Riordan to actually make it onto the warlord_party list atm, only the arihama_soldiers list. And only the warlord_party list is checked when setting riordan_askforhelp to 6.