The Hammersmith & City / Circle lines don’t give you any stations/stops/timetable data if you set it up for the outbound direction (?direction=outbound).
https://api.tfl.gov.uk/Line/circle/Timetable/940GZZLUGPS?direction=outbound - This has no stations, no stops. It does have the timetables & the Interval ID. But the array that would normally tell you where the train is going is empty.
Though if you set the direction to inbound:
https://api.tfl.gov.uk/Line/circle/Timetable/940GZZLUGPS?direction=inbound you will be able to look at the stations, stops & the Interval ID’s.
This problem exists with the Hammersmith & City line also. (There may be others, but I haven’t seen any yet).
Also the Circle line only has the Timetable for Sunday (its the same on the TFL website