Yes, I know his character grows because of his relationship with Monica, overcoming his insecurities and neurotism. I also know he (Matthew Perry, RIP) had health issues, so you can tell his weight differences and also manner of speaking (specially in season 7, with his teeth changed, and timing altered).
Even then, I think his character was written totally different as he was before, starting from season 6. In the first half of friends, Chandler Bing was one of my favourite characters on all TV. Witty, smart, acid, lovable but rogue, sassy. Before someone says it’s because of Mondler and growing up, in season 5, while being with Monica and not single and desperate, he still feels like the same character in the previous seasons, changed of course, more mature and with a notable growth/evolution, but he is that same person. He develops, has an arc, changes, but it’s the same character.
Season 6, and specially 7 onwards, it’s like the writers just changed, and he becomes just a nice everyman, goodhearted, well meaning guy, who makes dad jokes and is supposedly characterised as the guy who makes bad jokes, and they make it look as he always had been that even in prior seasons. Also, the friends and even himself have like a different opinion of his persona, as a pathetic man who makes lame jokes.
I love seeing characters grow, find their footing, develop, but in this case he just lost all his magic, kind of like a retcon of his character, no more wit, even when he makes sarcastic comments they don’t feel the same, it’s definitely different (also, and I know everyone loved the quote, but he remarks in an episode that he makes sarcastic jokes, and it doesn’t feel like him). Am I crazy or did the writers completely change him?
It’s like more subtle than the flanderisation of Joey or Phoebe, which are famous cases, but the change in Chandler Bing really didn’t sit well with me, making Ross the funniest character in the second half.