One of the reasons he doesn't really slur any of the words on let's say THINFS compared to LOAP is cause I think that jon doesnt think about the past or Richie when singing THINFS which makes him sound so good.

Richie made the talkbox part of LOAP and Jon might be thinking (maybe not actively thinking about it while singing LOAP) about the past, richie, etc which could be the reason he sings LOAP so horribly. But before Richie left 2013 he didnt think about anything like that causing him to sing LOAP very good those years.

Think about when someone might stutter words when sad but not stutter at all when in a normal or happy mood.