If you encountered the bug during the “A Tempered Blade” quest in which returning to Mezzamere doesn’t complete the quest, try turning on Warmode and returning to Mezzamere. This marked the quest complete for me, and although Bladesman Inowari was gone for about a minute, he returned and I was able to complete the quest. I hope this works!
You must have been lucky because this did not work for me and I tried it on two different characters with Warmode on.