I’ve asked for clarification from MS on this one, as I’ve noticed Member Disputes being mentioned a bit more lately, but based on this article the member dispute process and complaint process seem to be the same thing.
If you take a look at that web page, you’ll see it refers both to member disputes and complaints hence why I think they’re part of the same process.
I don’t feel that it was accurate to say that we can’t investigate a member’s issue without a member dispute/complaint being raised, as MS will investigate all issues they received, in order to provide the right support and resolution, as my understanding is the member dispute/complaint process is there to make sure there is a path of escalation for issues that can’t be resolved by MS.
That being said, that’s my understanding of things based on the document I linked further up my post, but there could be more nuance that I don’t have, as I don’t have MS training like the team does 
If you’re able to share any info via DM about being told that we can’t investigate without a dispute, I’d be happy to take that back to the team as well, as I’d like to better understand the context in which it was said. Like I say, any member of MS would investigate an issue raised with them, as that’s super important in order to resolve any member issues they receive.