The thing about Prayer.
He might struggle with hitting the notes PHYSICALLY.

He struggles to pronounce the words due to overthinking or something else with the mind (Not at all saying he is crazy or something)

Prayer also has a thing, the talkbox.
Jon hears the talkbox each time he performs that song causing him to think about the past, Richie or something.
The struggle and him failing to pronounce words is MENTAL but might FEEL physical.
(The way anxiety causes chest tightness for some people)

He performed LOAP easily 2010 cause he knew Richie and everyone were there and didnt think that richie would leave.

To be honest here when i have gotten sad (If someone i knew died or something) or angry my voice suddenly got pretty weak and sometimes i talked and ended up exactly like jon choking up in Vegas Runaway Tour 2018. But once i stopped being angry, sad or whatever i was my voice went fully normal.

Jon in interviews talks normally cause he isnt thinking about richie or anything.

Now the PHYSICAL struggle might be hitting the correct notes like G4, etc.

Vocal cords: Note/Pitch. Darkness or lightness of note (woman hitting a note sounds different from a man hitting the same note.)

Mouth, tongue, teeth, lips: For pronunciation itself.