The Problem
When using, cloudbuild.yaml files, specifically named for their build environment, such as cloudbuild-dev.yaml and cloudbuild-prod.yaml, and configured/targeted in the Trigger settings they aren't found/recognized when GCB reacts to a GitHub event (push etc).
However, it's working just fine when manually running the Trigger from GCB console.
When using an ordinarily named cloudbuild.yaml, in the root of the project, Cloud Build correctly runs the expected steps.
The Workaround
In short, there isn't an easy one (imo). But to get it run you need to use just a single _cloudbuild.yaml).
However, to effectively re-use that for both dev and prod environments one is blocked by this issue