Every 5 hits the debuffs get extended by 1 turn, but when the 5th hit is the same time as Fire Surin's S3 buff extension, it doesn't stack . It's not getting resisted or anything. It never gets the double extension from S3 + 5th agree. 

Unless I'm missing a detail that explain this, it makes no sense. 

Similar thing happens to debuffs to should reset their turn count from 1 to 2 turns. It as though the debuffs don't actually proc.