Dr.KarenChen said:

Resolution therefore closed the doors unevenly, and the asymmetry follows from the bulks lists.

This is where I part company with the consensus forming above. A research-chemical supplier selling lyophilised powder labelled research use only is not compounding and is not claiming to. It is a different legal universe with no pharmacy oversight, no patient relationship and no content guarantee, and conflating the two in these threads helps nobody.

One concrete data point for the thread. Worth knowing that the injection site changes very little. Abdomen, thigh and upper arm are bioequivalent for semaglutide, so a site change is not a plausible explanation for a bad week.

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

Dr.RaviCardio said:

A research-chemical supplier selling lyophilised powder labelled research use only is not compounding and is not claiming to.

There is a second half to this that has not been said yet. The dose-response is real but shallow at the top. Across STEP 1 and STEP 4 the gap between 1.7mg and 2.4mg is a couple of percentage points of body weight on average, and the average is carrying a wide spread — plenty of people at 1.7mg sit above the 2.4mg mean. If a dose is working and tolerable, "working" is the relevant variable, not "maximal".

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

Why a shortage listing created a legal pathway at all, since a shortage is a supply fact rather than a permission?

Closing the loop on my own question.

The bulks-list asymmetry was the piece I had missed entirely. It explains why one of my two pharmacies is still arguing it can supply and the other simply stopped.