Håkan - always good to hear from you.
This issue was surfaced by an internal test case.
We try to save an object to a never-been-used collection name. Before we STORE an object we try to DELETE it. If an object was never stored in the collection, previously we got B4xx01yy on the DELETE, which CMOD treated as "doesn't exist" and proceeded with doing a STORE.
After the OAM compatibility PTF we now receive B4xx04yy and the plan was to add that to the list of reason codes that means "doesn't exist".
However, OAM development is currently looking at this and CMOD is "standing by".