Since I upgraded my MacBook Pro to macOS Sierra, two keys seems to have been switched around in the keyboard when I use my external keyboard. 

I use the Spanish ISO layout, and the "º" and "<" keys have been swapped. The laptop's keyboard works fine. I have not fiddled with the keyboard configuration, other than changing to my regional layout and swapping modifier keys (it's a windows keyboard).

It worked flawlessly<sup>1</sup> before the upgrade. I know very little of how to troubleshoot macOS, what should I do?

<sup>1</sup> I just remembered that, no, it wasn't all that flawless; from time to time the keyboard would stop responding, and I'd have to unplug and plug again. Don't think it's related, but...

PD: It happens with all external Keyboards. Tried with a few. All of them where ISO, and it seems the problem is in macOS treating my ISO keyboards as ANSI; the laptop's internal keyboard is ISO though, and has no issue. I've done a clean install of Sierra, and it's still wrong.