Ah, I see, you meant the bounding box center handles. Then ignore what I said about the center in the Attributes :)
Yeah, you are dealing with a GPU bug, and I so with you had it recorded on video. If it happens again — try to catch it, and supply the full OS and GPU specs, along with some background info about the steps taken and ideally a test file. These are hard to catch in-house.

However, I stand sure the bounding box gets hidden when a guide is being dragged... the 'snapping to a ghost' part is clear, but I’m not sure center handles can be visible... can you show it to me, please?

Thanks for checking this. I think I may have caused some confusion by calling them “bounding box center markers.” I was referring specifically to the square midpoint handles on the bounding box — the handles centered on the top/bottom and left/right edges — not the object's center point controlled by the Attributes panel.

I checked the Attributes setting anyway, and the object's center point was already enabled. While testing this, though, I found something that makes me think I'm dealing with a display/rendering bug rather than normal guide behavior.

I moved an object that had been interfering with Smart Guides and Illustrator left a visible “ghost” of the object in its previous position. The ghost could not be selected and Smart Guides did not recognize it. Toggling Preview caused the ghost to disappear immediately.

More importantly, after toggling Preview, the original problem also went away: when I drag a guide from the ruler, the bounding box and its midpoint handles now remain visible. I can therefore use those handles to position the guide as I expected.

So I think you're correct about the normal historical behavior you described, but that doesn't appear to be what I was seeing. Illustrator seems to have gotten into a stale display state, and forcing a redraw by toggling Preview fixed both the ghost object and the bounding-box behavior.

Alan, the bounding box vanishes always when a guide is dragged out from a ruler, only the objects’ edges stays visible (if you have them turned on, actually). To be sure it never changed, I checked a handful of builds up to CS1.

But there’s an explanation.
Objects in Ai have their own centers (usually bounding box obscures them with its heavy dot in the middle — that’s why I prefer to never use the bbox). Shapes like rectangles and ellipses get these enabled by default (and keep them even when they become expanded or modified, unless used in a Pathfinder operation), but freeform created things, like ones made with a Pencil, don’t get those — and there’s nothing to snap to when you drag a guide.

But we can toggle these centers. Open Window > Attributes panel and make sure it’s expanded (click that up/down arrow next to the panel’s caption). You’ll see 2 buttons there, to enable or disable the center. Does it change anything for you?

If it matters, you can also review and upvote this request:
Allow to disable or enable center points for all or specific objects types on creation
https://illustrator.uservoice.com/forums/333657/suggestions/47663531

or that one
A preference to not select objects by center point
http://illustrator.uservoice.com/forums/333657/suggestions/35732254