I also tested subfolder in my lab, the subfolder in Inbox doesn't sync with Outlook:
- I created subfolder in OWA, it doesn't sync to client 1 and client 2 by default.
- I created subfolder in client 1, it doesn't sync to OWA and client 2 by default.
But, for the folder created in OWA, you could choose it from the "IMAP Folders" configuration, then this folder could sync from server side to Outlook client: