See title. I can add it to my character inventory just fine. However it seems that searching for it come up with zero results. Is this happening to anyone else?

Beyond's search kind of sucks. It is okay for a casual user, but the more you get into the game, the more you realize how unreliable it is.

If you know the subject you are looking for is in the Basic Rules/SRD, you are better off using Google and type "dndbeyond" in their search along with whatever you are looking for.

Around here it’s not considered unusual to forget the search at even exists a ten not bothering to wast time using it after so long. If you want to search for a specific magic item, it’s better to search directly in “Rules->Magic Items:” (https://www.dndbeyond.com/magic-items).

There is a direct “Rules->[Whatever]” page for each category of player and DM option: Races, Class, Backgrounds, Feats, Spells, Magic Item, Monsters, Equipment, and Vehicles.

Assorted items or sets with a shared name, like the various Ioun stones or the figurines of wondrous power or the instruments of the bards, are often sorted out individually in the equipment tab of the character sheet, but grouped together in the Magic Item search. There's no way for anyone to know this ahead of time, and it's definitely something that ought to be fixed (but if there are plans to do so, they're rightly not a priority). But it's a good additional tip when searching for items. In this case, searching the magic items list would only bring up the Ioun Stone of Mastery if you search for 'Ioun' and click into the main entry for 'Ioun Stone.' It's especially puzzling for Ioun Stones since some have their own entries in the search. But, that's where it's located.