Managing personal Outlook.com account settings, security, and privacy
I have the same issue and raised a case via chat. After going through standard troubleshooting, turning settings off and on, logging out and back in, the agent suggested creating manual rules to filter by subject or content. I explained that I didn't want to do this, as the junk email subjects were varied and I didn't want to block legitimate emails in the process. They replied with this...
"We are suggesting you create this rule temporally as I have just checked with my supervisor and Outlook research and development team that I was checking with my resources and got to know that this is a general emerging issue affecting all Microsoft customers. Microsoft Engineers are working Tooth and Nail to provide a solution to this issue. Be rest assured that this is a top priority and will be resolved as soon as possible.We apologize for the inconvenient and appreciate your patience. We will keep you posted once fix is deployed."
So looks like a known issue that MS are working on. I asked about a timescale for a fix and they couldn't provide one.