When you unlock a new advancement tab (by completing any advancement on that page), the new tab is placed next in the list of tabs, putting the tabs in the order you unlocked them. However, whenever you reload the world (either via the /reload command or by quitting and reopening the world), the tabs get reordered into a completely arbitrary, but more or less consistent, order. Each time you reload, the order will be the same, but the order doesn't seem to be based on any information from the root.json files. It is not ordered alphabetically (by folder name or by display name), by date created, by number of advancements under the tab, by alphabetical order of the advancements under the tab, by alphabetical order of tab icon item names, by creative menu order of tab icon items, or by the background image. I cannot find any rhyme or reason to the ordering I get. (The ordering I get for the default tabs is Nether, Adventure, End, Husbandry, Story.)

Side note, I created a datapack on that world with a single root advancement included in it, and that advancement tab ended up between the Nether and Adventure tabs, rather than at the end like I was expecting.

Also, while this is purely circumstantial, I looked up some screenshots of the advancement menu from other players, and the screenshots all had different tab orderings. This could be because they had all just unlocked the tabs and hadn't reloaded their games, but I find that very unlikely.

What I expect to happen: For the tabs to either be permanently sorted by the order you unlock them, to be sorted alphabetically, or to have a key in the root.json file that lets the datapack maker decide a priority value.