Call me crazy, but I think Jon may have had a vocal cord surgery around early 2016 that simply didn't turn out too well. He still had his tone and (low/mid-range) pitch control in 2014-15, wasn't cracking nearly as much as nowadays, and so on. Nowadays his ability to stay in tune and follow the melodies has declined so much that at times it's hard to believe he once was a great singer. He doesn't only struggle with the high notes (which actually seem easier for him than the lower ones, as hard as it is to understand), but his whole voice has become hoarse, monotonous and toneless. It simply doesn't resonate anymore.

Jon no longer sounds like himself. Hell, he can't even shout while speaking anymore.