Adding the numbers, since they settle part of this. The four things to check on any report, in order: does it name your batch, who commissioned it, what method was used, and does it report content as well as purity. A report failing any of those is weak evidence regardless of the percentage on it.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

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

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

Dr.NateNeph said:

The four things to check on any report, in order: does it name your batch, who commissioned it, what method was used, and does it report content as…

Coming at Dr.NateNeph’s question from a different direction. The provenance questions are more useful than the numbers. Does the report name the batch on your vial. Is the date consistent with when that batch existed. Is it addressed to the vendor or to the person who paid for the test. Was the sample taken from general stock or supplied by the vendor. A perfect certificate for a different batch is worth nothing, and vendor-supplied samples are the most common way a genuine report ends up misleading.

OP back with an update, since a thread like this is useless without one.

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.

Dr.RheumBOS said:

The provenance questions are more useful than the numbers.

Right, and one flattering data point from a group buy is not consistency. Consistency means separate batches, separately commissioned, over months.