Some helpful info with InterfaceOptionsFrame_OpenToCategory
 

This is used to display a specific addon's settings in Menu -> Interface -> Addons...

Call this function twice (in a row)... There is a bug in Blizzard's code which makes the first call (after login or /reload) fail. It opens interface options but not on the addon's interface options; instead it opens the default interface options. If you call it twice in a row, it works as intended.

Yep. It's been like this for years. I don't recall if this is still the case, but I think that for a time it was only this way if your addon was out of view in the left scroll frame (the list of addon pages in Interface Options).