I've decided that the possibilities of formatting for a 3rd person's POV are points along a spectrum. It's the only way I'm going to be able to distinguish between different practices.
Things aren't just black and white in a spectrum. Shades of grey, and colours are important. It's the reason I find them very useful.
In the POV situation, one end of the spectrum is 3rd person omniscient. The other is the most severely limited 3rd person. This is where a character's own experiences, and all actions by the supporting cast, as observed by the primary character are formatted as all his/her own. Where the supporting characters get only to say things. All else is part of the primary character's experience.
A little like in Cormac McCarthy's The Road (2006). Which is a gripping read. But minimalist in punctuation. Capital letters, full stops, the odd question mark. Even an apostrophe -- on page 107, in 'But we're not dying.'
Often the boy's experiences are paragraphed as part of the man's. As on page 113.
There were mattresses and bedding arranged on the floor in front of the hearth. Papa, the boy whispered. Shh, he said.
On page 114 on the other hand, the boy gets his own agency, in a new paragraph.
...a large padlock made of stacked steel plates. He stood looking at it.
Papa, the boy said. We should go. Papa.
I haven't yet worked out the reasoning for the two different treatments but with McCarthy one gets the feeling there will be one.
One definition of 3rd person limited I've come across -- I've still got more pieces of paper in my house than data on memory sticks -- states that, 'third person limited is the inner and outer world of the main character and the outer world of the rest.'
I take it to mean that 'the outer world of the rest' is being described from the point of view of the main character. (The main character can't know the inner world of his fellows.) But theres's no mention of how this should be formatted. No recognition that describing 'the outer world of the rest' from their own point of view -- how this is usually formatted -- might sound like head-hopping.
The only thing even partway definitive on paragraphing that I have found, is by Sarah Endacott of Edit or Die in her Style and grammar Notes, page 12, Speech or Dialogue ---
"starts a new paragraph. This must occur when there is a new person speaking, and should occur if the person talking commences a new train of thought, his/her own new paragraph. If in doubt, start a new paragraph. For every new speaker start a new paragraph. Never have two people speaking in the same paragraph, even if they are interacting."
While "If in doubt, start a new paragraph," gets me over the hurdle, though I still don't know why that hurdle.