I always seem to get these very hard to remove rings around the pots & crockpot i use to brew in. They dry as the water level lowers & look almost powdery, but remain stuck & very hard to wash out.
The rings are accompanied most of the time by a very thin film that floats on top of the brew & does not separate.

I can't find much info on burnt or carmalaized brews to compare to. I use low heat & make sure not to let the brew even bubble the entire time.

Has anyone else encountered this while brewing? Is it normal or is there any ways to avoid it from happening?

It's normal. Just scrape the sides a bit with a wooden spoon if you want to get it off.

Don't be afraid to put the brew at a full rolling boil. Simmering isn't necessary, it can take it. The chances of burning it is mostly when your reducing the final liquid down to drinkable amounts, and in any case it doesn't really seem to effect the overall potency, just the texture and taste in my experience.

scraping it off isn't a problem.
I was more worried i'd been burning off actives.