The Royal Victorian Order was set up by Queen Victoria (and named it after herself, no less🤣) as a means to recognise service to the monarch. It is I think the only award where the decision as to who gets a medal/rank within it is entirely the decision of the monarch, not the government. So, members of the various ranks of the order include private secretaries, holders of key palace roles like the CFO etc, and long-serving palace staff (“Tall Paul” the late Queen’s footman who walked in her funeral procession, for example, and Angela Kelly have all been made members of the order, along with many others). Plus family members who aren’t made a member of the order of the Garter (which is the higher ranking award, but is pretty much limited to the monarch’s immediate family and not spouses, other than the spouse of the heir). So, Sophie and Kate are members of the Royal Victorian Order, as is Harry too.
Every four years they hold a service at Windsor for people who are members of the Order to attend. So it’s a bit like a regular alumni gathering and drinks, I guess, though with a church service attached. Only the highest ranked awards get an actual robe, as they are akin to a Knight or a Dame - most recipients get a medal they can wear.
Given it’s a private award, and the gathering is a private event, I can see why Prince Andrew’s attendance wasn’t officially noted. Given it was also the first one that has fallen after the Queen’s death, and almost al of the people attending would have received their award because of their service to her, I can see it being a bit of an event of remembrance and reunion of the late Queen.