[gus_massa]

I agree. The original explanation is a good guess but it's not what is happening in the OP.

There is experiment to detect some sugars, that is somewhat similar to this alternative explanation (and unrelated to the experiment in the OP). More details in:

https://en.wikipedia.org/wiki/Tollens%27_reagent

Random quick video: https://www.youtube.com/watch?v=21PHoxlukME

NileRed video with more technical details: https://www.youtube.com/watch?v=nGmxHLHyUPc