I'm using Plasma 5 on Debian testing and so far I very much like it, it runs rock-stable too. However, I noticed some weird behaviour of sound settings: it appears that volume for my device, which I would like to be set to 80%, gets restored to 100% on a seemingly random basis.

Fore example, today I launched Spotify and noticed audio is on 100%, so I lowered it to 80%. Then, after some time, I closed Spotify and started a movie in my browser. After the movie finished and I closed the browser, I noticed that audio is back to 100%. Attempting to reproduce it this way yielded no results, though.

Are there some bizarre rules that make this happen or is it just an elusive bug?

That is probably a bug. The sound levels are suppose to change according to the device you are using, so if you use a headphone for example, you will get a different associated sound level compared to speakers. Are you plugging and unplugging your headphones?