That second source goes on to point out that its major concern (in terms of "uncertainties and critical risks") is healthy people using medical masks in places

That second source goes on to point out that its major concern (in terms of "uncertainties and critical risks") is healthy people using medical masks in places where there isn't enough supply and depriving healthcare workers of them, like I said. It specifically does not advise against nonmedical masks, and simply notes that the WHO was still actively studying their use at the time and would make a recommendation when evidence was available. So the "flip-flop" here was the WHO saying "I don't know, I'm looking into it," and then later saying "I've looked into it and have some strong recommendations."