Just ran an experiment on preparing mimosa tea with a large phosphoric acid (enough to bring tge soup to pH 2 when reduced to 1.5L), and upon adding NaOH, the solution turned gray as a huge amount of gray sludge precipitated out of solution.

https://i.postimg.cc/s28MVHyZ/20220726-181455.jpg
https://i.postimg.cc/90Hz94Dm/20220726-190246.jpg

I have observed that this gray sludge forms when using strong acids like phosphoric or sulfuric, but does not form when using vinegar for the acid boil. The gray substance can be eaaily filtered out. This gray sludge I am assuming is full of tannins. One theory is that high pH makes the tannins highly soluble in the water, so when basifying, there are a lot more tannins that precipitate out.

In this test, a HUGE quantity of this gray precipitate was filtered out, maybe 300g of it. Adding vinegar to it redissolves it and changes back to a brown solution.

Has anyone here observed this phenomenon and has a good understanding of what it is?

Sounds, and looks, more likely to be an inorganic phosphate. These will precipitate with alkali and redissolve with acid because acid phosphates are more soluble than basic ones. Magnesium phosphate is very easily precipitated with ammonia, for example.

A brief calculation based on the amount of phosphoric acid used should be instructive in this respect, when compared with the amount of bark used - which was how much MHRB, incidentally?

The pure phosphate is nominally white but the grey colour arises through adsorption of the dark coloration from the basic MHRB soup. I bet you could get nice white crystals by cleaning up the brown vinegar solution with activated charcoal, filtering then slowly adding concentrated ammonia (if you have magnesium phosphate) or just repeating the NaOH precipitation if procuring ammonia seems like too much trouble.

Tannins would be soluble in alkali.

The precipitate is presumably insoluble in naphtha and other [neutral] organic solvents?

What actually happened here is the tannins precipitated out at pH of ~8, but at higher pH they redissolve

OK, that sounds more like the behaviour of tannins, although is something you could have mentioned in the OP. It would still make sense to carry out further analysis on the precipitate before jumping to conclusions regarding its chemical identity.

Incidentally, phosphoric acid is not counted as being among the strong acids.

Also,
Did you mean low pH here? Adding base increases the pH, while adding acid lowers it.

Have you monitored the pH changes during the precipitation and redissolution process? do you think it's likely that DMT could have co-precipitated at any point?