There's absolutely nothing in ISO9000 to cover this. It's just one of those things that SLAGIATT (seemed like a good idea at the time), became standard practice, and has grown to be almost axiomatic.
I think it grew from formal auditing practice, where a Major nonconformance was enough by itself to warrant refusal of certification/registration, a Minor was enough to insist on corrective action (and if there were enough minors, refuse certification), and an observation was not one where corrective action was mandatory.
However, given that it had to do with what was allowed and mandated, it has little relevance in any responsible approach to internal auditing or classification of nonconformances. In internal auditing, there are either things that need to be fixed or things that don't: the things that need to be fixed can be regarded as problems, nonconformances, issues, or improvement recommendations. In classification of product/service nonconformances, I suggest only two classifications: acute, where immediate corrective action is warranted (we never want this to happen again), and chronic, where Pareto prioritisation should precede corrective action (this is happening too often).
An observation is essentially an OPINION. Read this thread (https://elsmar.com/level2/m-vs-m.html) for some thoughts on what an observation is -- If you've never heard of a LOOK ( I hadn't), it's also discussed in the thread. This thread also has some oblique references. When I see an auditor write up an 'Observation' I ask myself this: "Is this person qualified through experience, etc. to be offering what is no more than their advice to me on my business and/or process(es)?" Double check with your registrar -- Ask what their expectations are when (if) they write up an Observation. Some say you can ignore it while others expect the Observation to be addressed in some manner. I have heard a registrar tell the client that they expected the observation to be addressed and action implemented by the next visit!
ISO 19011 - Quality and Environmental Management Systems Auditing Forum Discussions