This happened to me a couple of days ago when I was training in the Castle Corridor [7]. https://bbb.hidden-street.net/map/mini-map/zipangu-castle-corridor-7 To replicate this bug, just FM in the middle bottom close to the hidden portal to Castle Corridor [4]. Once you come back from FM, you'd fall down and get trapped in the hidden portal, unable to move/jump. I had to use some return scrolls to get out.
Interesting bug, but I just tested it and relogging/changing channels fixes this without the need of a Return Scroll.