First may I ought to make clear that I'm talking about close 3rd person PoVs, not any other type of third person narrative. If someone wants to write a story/novel which isn't close 3rd person PoV, then they can head-hop as much as they like without necessarily breaking any conventions.
If all one does is to add the blank line (between the PoV of Character A and the PoV of Character B), you'd have a point. But what comes after the blank line would be different from what would be there if there was no blank line. The blank line is merely there to signal to the reader that they may ("may", because the scene break may be between two scenes from Character A's point of view) need to read what comes next in the context that the PoV has changed to that of Character B (what they can see hear, smell, taste, remember, plus all the prejudices and knowledge they have their heads). Adding the blank line is by far the easiest bit of the process. The next easiest is identifying the PoV characater. The rest is harder.So If mid scene I put one space/divider/break and switch POV and stay with that to the end you seem alright. But if somehow the space gets lost suddenly its all wrong.(I had this happen when the publisher created the ebook and it was rather annoying but I've yet to be hauled out on the rug to be persecuted for it.)
But if the switch is meant to go unnoticed, why do it? What would be the point? Surely it can't be simply there to slip something into character A's PoV about which Character A is totally unaware.Now if it was hoping back and forth then maybe we could object. But there are still some scenes when things are really close with two people (does not have to be romance) when it is easy to make that switch once over once back and almost have it go unnoticed. If you do it just right. But in the long run if I put some break or divider between the switches then it becomes more evident and actually draws the readers attention to it before it's happened.
I'm not sure where you're going with this, but it isn't really about formatting, is it? The frequency of head-hopping has nothing to do with whether there are scene and chapter breaks or not. Head-hopping is dragging the reader back and forth between heads as the writer sees fit, which rather undermines the idea of close 3rd person PoV (which is to put the reader in the head of the PoV character precisely so that they experience the scene with that character). If the PoV changes from paragraph to paragraph (or sentence to sentence) how strong a bond is the reader going to make with the supposed PoV character? Who really is the PoV character in this sort of narrative?It really becomes a style issue of; do I want to take a risk of jarring them with the POV change or do I want to seriously jar them in and out twice with the obvious break.
The thing is if you have no chapters in your book then you would quickly see all the head hopping you do. If you have no dividers or breaks then it becomes more evident that serious head hopping is going on. So it all boils down to formating and styling not so much rules except perhaps a rule of etiquette in not wanting to piss off the readers.
It isn't about sensitivity in the general sense; it's that head-hopping can, if not handled expertly, undermine the "close" in close 3rd person, and some people obviously notice that more than others.In the same token it might do them a favor to on occasion forget etiquette for a rather tense scene that doesn't need a full stop in the middle for someones sensitivity.