A narrower follow-up, since the general answer is now clear:

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

josh_phd_bmore said:

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality…

Pushing back on josh_phd_bmore here. 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.

Ask again with the specifics and you will get a better answer than this one.

josh_phd_bmore said:

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality…

Agreeing with josh_phd_bmore, and the qualification matters more than the agreement. Yes, and the corollary is less popular: if the reasoning holds, it also holds where it produces an answer we do not like.

josh_phd_bmore said:

Went back through my own correspondence with an earlier supplier and the warning signs were all there in the replies, weeks before the quality…

This answered a question I did not know how to ask.

From the other side of the consultation, briefly. It helps to ask what evidence would change your mind before you look at any. If nothing would, the discussion is not about evidence, and it is better to say so early than to spend nine posts discovering it.

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