The issue is when these double-sided materials are added to the document (whether via the Add Material button or via scripting), if the material is duplicated, it instances the material (i.e., the material’s name gets a number added to it), but it does not instance the front and back materials (i.e., it does not add a number to the names of the front and back materials of the duplicated material). The result of this is that when the front or back materials of the duplicated material are edited, the front and back materials of the original double-sided material are affected, too. I suspect both the original and the duplicated DS material are referencing the same instances of the front and back materials.
To be clear, so far as I can tell at this time, this only seems to affect some materials that I previously created, edited, and then saved. So even though I edited and saved them a few days ago in the materials panel, they were originally created months and months ago. This issue does not seem to affect newly created materials…even if they seem completely identical in every way. So, obviously, I guess I just need to re-create all my materials from scratch and just be glad I don’t have more than 16 materials (really 32 since they are double-sided).
But just in case the devs are curious, here is a sample material that is affected by this issue.
Gem - Stylized - Amethyst.rmtl (19.9 KB)