Whelp, I've burned out my Power Tag as well. It's a bit odd, the portal continues to work with retail Amiibo so it's definitely the tag, however the tag can still be read by the developer and homebrew apps on the 3DS. I can even format it, write app data, register a Mii, etc. It just isn't seen or read by any games. Looks like the tag inside the Power Tag is fine but whatever additional circuitry is in there is busted.