I updated everything again with sd card format, nvram clear, flashed directly kof98, I can't reproduce it now.
I am pretty sure I was on last firmware and last menu already according to the date, but I noticed something during sd card check, I was missing the menu files in the update folder, it is my mistake but I don't know if it could be related to this bug.
Another thing, I have flashed quite some games since last nvram clear + buff file, how often is it needed to delete buff file + nvram clear ? I didn't try this for some weeks from now since the Last Blade 2 testing.