I've left my mimosa brew to sit at the back of my closet for about a week, since I didn't have a chance to drink it. This is the tea I made from 25 g of wood and lemon juice which I reduced to ~200mL from about 500-600mL. I've noticed that the brew has left a thick, black residue on the jar, and I know it's probably not plant matter because I filtered this tea three times. I can only assume something is falling out of solution (and hope it's not the good stuff).

Is it bad to leave these brews on the shelf?

These are probably the tannins. If so, it should taste better now.

I had residue like that when reducing well-filtered tea and then cooling the reduced tea over night.

That's most likely right, tannins. The soluble tannin precursors slowly oxidise and polymerise, leading to the observed precipitate. It's possible that some actives stick to the precipitate, one way to check would be decanting away the supernatant and adding base to the precipitate, followed by extracting into NPS, etc. The amount is likely to be insignificant if you have added sufficient acid.

DFZ, describes the likely. Not a new observation. Filtered, then reduced MHRB tea, stored in the fridge, yields...precipitate + supernatant. Many reference the "sludge". My observation has been that with adequate tea above the sludge, no matter. If one is poor on actives, the sludge may add a bit.

Kinda silly really, with adequate RIMA, even modest doses of enteral tryptaminics...BOOM!

Peace