Old skill strengthened from completing new lesson
Haven't seen this posted, so figured I'd report it.
I recently completed the 2nd lesson in the "Present Perfect" tense for Spanish. After finishing the lesson, "Countries" was strengthened (I had completed countries already, but it needed strengthening). This bug occurred on the desktop version.
The thing to understand is that the whole gold/decay mechanism is based not on skills, but on words. Each lesson introduces a certain number of new words. Each word has a strength associated with it -- Duo's estimate of how well you know that word. When you use a word correctly in a question, the strength increases. When you use it wrong, it decreases. Also the strength decreases over time.
When the average strength of the words in a particular skill goes below 80%, then that skill decays and is no longer gold. The algorithm isn't really measuring your general knowledge of "verbs" or "present tense" or "countries." It is simply a measurement of your knowledge of the words that happened to be introduced during that lesson.
It can lead to some counterintuitive behavior, but mostly works fine. I suggest not worrying too much about the algorithm and just continue on.
I posted the same thing about a week ago here for the Turkish course (the search function here on the forum sucks, IMO).
I was told it is normal Duolingo behaviour.
For what it's worth, the gilded, unlrelated skill turned back to blue within hours, so I don't think it matters all that much in the grand scheme of things.