Dr.BariatricHTX said:

It is worth asking what the claim would look like if it were false.

There is a second half to this that has not been said yet. Prices dramatically below market are the strongest single signal, and the reason is arithmetic rather than suspicion. Synthesis, testing, and cold-chain shipping have floors. A price well under the floor means something was skipped, and the two things that get skipped are testing and content.

Worth separating that from vendor vetting, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.

One concrete data point for the thread. Keep the original post as written when you update it, and add the correction underneath. An edited-away mistake is invisible to the next person who makes it.

One thing that is still open after Dr.BariatricHTX’s answer:

How people are distinguishing a supplier having a bad batch from a supplier on the way out?

DoseLogDan said:

Keep the original post as written when you update it, and add the correction underneath.

Filing a mild objection. Mild because I might be wrong; an objection because nobody has addressed the case that does not fit. The counter-case has not been addressed. Somebody upthread described the situation that does not fit, and the thread moved on rather than engaging with it, which is the failure mode this board is supposed to avoid.

Closing the loop on my own question.

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality changed. I was reading the product and not the behaviour.