*I didn’t see this question @xavier – it’s interesting. I know what you mean, it’s these little “Stoefchen” in Germany: 
You’re right the tea tends to get pretty “heavy” and dark. But then that happens to tea sitting in the pot, even unheated. When I make a pot of tea, and leave it standing, the brew always looks darker a couple of hours in. Even with filtered out tea leaves. You do mean, without tealeaves, it still tastes bitter?
The evaporation explanation sounds plausible to me, some of the water evaporates, and leaves a more condensed tea behind. Keeping tea warm probably speeds up the process. Anyone else any thoughts?