You only have to look here https://www.financial-ombudsman.org.uk/decisions-case-studies/ombudsman-decisions
If you do a search for Monzo complaints that are NOT upheld you’ll see the FOS states quite clearly the bank has a right to close down accounts without giving a reason. They must give proper notice which I think 2 months is reasonable considering CASS takes 7 days.
I’m not saying I agree with what’s going on here but I certainly wouldn’t expect the FOS to to back a complaint based on the information provided so far