Right, but is it the right mental model to demand a root cause to be delivered as the outcome to the investigation, when there are lots of things going wrong and process & architecture problems that make the involved failures fatal.

This follows Google’s published post mortem template: https://sre.google/sre-book/example-postmortem

What you’re saying can go into the lessons learned section.

That template is a bit better wrt the cognitive problem in question since the heading is "root causes" (plural). So it's more compatible with the epistemology that there are lots of interacting evens and properties and doesn't frame your thinking around pinpointing a single event in the complex graph of interacting processes leading to a bad system state. (it's more naturally a graph than a tree whcih is implied by the classic RCA model)