I’m not using V3, but indeed it seems there’s no “Simulate uplink” in the web interface, nor any test input for decoders. I’d say the decoders would be the same, but your decoder above would not be valid in V2 either; it’s missing the closing }. Indenting your code helps a lot to see that, but also the editor will show you these errors, in both V2 and V3.
(Also note that toFixed gives you text, not a number.)
As for:
Note that it says “Resets Join Nonces”. So, I’d say it’s something that the Join Server may, or may not, do. That said, I’ve no idea.