Weird Outlook/IMAP isssue

[rusky]

So I'm a bit stumped on this so I'm hoping the hive mind can help!

I am using Outlook for my work emails, these sync to a cloud platform that does support IMAP. I try to keep my inbox tidy by filing emails in folders.
Initially, I created subfolders under the inbox (as I have done at previous places) but these were appended with "(this computer only)" & didn't sync with the cloud platform.

Today, I realised that there was a folder under the email account, not the inbox that had synchronised so I moved the other folders there but only a handful of the folders have synchronised.

All the folders are now under \\user@domain.com but the type is either "folder containing IMAP items" or "folder containing Mail and Post items".
The odd thing is, on the handful of folders that have synchronised with the cloud platform the folders are of both types yet other folders that are seemingly identical have not synchronised.

[Kim]

I know very little about Outlook, but I do know that whether IMAP folders support sub-folders is a function of the IMAP server configuration, and restricted by the underlying system it's using to store mail.  If it's using MBOX files to store data then it's likely limited to storing emails or folders in a folder, but not both.  The inbox is often singled out for special treatment, because reasons.

I'm sure Outlook handles these sorts of limitations with all the grace and clarity that you'd expect from the Mega-Global Vendor Lock-In Corporation of Redmond, USAnia...

[rusky]

I'm ignoring the inbox as the server doesn't seem to support subfolders.
I can't see why only a handful of the folders that are under the mailbox are synchronised. I don't have the option to subscribe to them either yet all my folders are in the same location.