So I figured out the issue: it was a naming collision in the supervisory tree.
I had a regular Supervisor created in an early step and given a certain name. Later, in a different module, I accidentally created a DynamicSupervisor with the same name.
This is exactly what was happening. Because I was calling DynamicSupervisor.start_child/2 while using a name that was also associated (and associated first) with a regular Supervisor.
The extrace utility eventually got me to the code that Matt had called out, and that was where things were failing. It wasn’t clear that there was a naming collision, but stepping back with all of the information I traced through the whole process and across modules and with fresh eyes I saw the collision. This is all test/development support code that wouldn’t be running in production and so I’ve taken liberties and short-cuts here and there… and in this case it bit me. <sigh/>
I wish I could mark two solutions because both @LostKobrakai and @al2o3cr really contributed to getting me across the finish line with this.
Thanks to all that commented!