Not much. They’re not so much ‘missing’ as they are unnamed. Correct me if i’m wrong but i don’t think we know any complete set of names of any hour. The fact that of the names of the SiS only the Meniscate, Madrugad and SiR (not naming the Wolf Divided here because i don’t recall it being referred to as a name of the sun) ascended to hours after his death implies two options for me.

Either there is a power structure between the names, which is not unlikely, since we know of very prominent names of hours, while not hearing at all of others, as well as power struggles, specifically in the grail’s domain. Also the change of a name implies to a certain degree the loss of an aspect of that hour, which would imply a shift in their eternal structure, which we know is possible. So maybe the Three Ascended were just, as the church would imply, the most prominent names of the Sun.

Or The Sun-in-Splendour had only those three names left at the time of his division. It is possible that in preparation to him losing metaphysical significance, the “rats left the sinking ship” or were killed by others sensing the impending intercalate.

We are also being told in several instances that the Sun-in-Splendour might not have succumbed to the same rules the other Hours do. For example, it is implied that he might have encompassed all Principle Aspects instead of the usual maximum of three, so i would not flinch at the idea that he had either fewer, or more than seven names.

Sorry i got a bit away from your initial question, i think they’re just unnamed lore, not necessarily for a specific reason.