Using Classic Outlook on Windows for personal email, calendar, and contact management
Hello
The rules are indeed designed to apply to the Inbox only. The junk mail folder is technically being treated like a "deleted items" folder, where emails would be deleted automatically when they are 10 days old, whereas, emails are deleted 30 days old in Deleted folder.
If you want your rules to apply to Junk folder, you would have to set it up, and use the "run now" option, then on the next step it would allow you to choose which folder. That's really the only way to apply rules to junk folder as of now. Running it automatically is not possible.
Sorry, I know this is not exactly what you want to hear. You can consider voicing your feedback via the Feedback portal so it gets to the product team. As forum user, the best I can do is just to inform.