The Drax Cave/Categories disappearing after adding hidden cat for Game of Rassilon

I spent some time last night creating some year pages and noticed a weird little glitch happening. When I added a year page to the Years category (per Game of Rassilon) all of the other categories on the page would vanish. The century category, the decade category, even an articles needing citation category in one case. If you look at the history for 1847, 2604, 2605, etc, you'll see that immediately after I add the category years I have to re-add the categories that were already there in the previous edit. I don't know if this is because I was adding the categories using the category module, but this has never happened before and seems weird. The articles needing citation category came back easily, all I had to do was edit the page and make no changes. This seems to have been purely a visual thing (I couldn't see it even though it was there), because the category is never missing when you look at the source through the edit history. But the year categories (century and decade) definitely vanish from the page and have to be re-added.

Also, I noticed that for some decades I had to create the decade category page. Is there any harm in creating those category pages now even if we don't have to use them yet?