Some of the overnight TL services were shown as "Q" (runs as required) paths long before the current situation, where almost every train throughout the day currently has a comparable "Q" path for running as a "*T**" headcode service via London Bridge vice any of the longer diversions on routes through Elephant & Castle.
By and large the overnight passenger services between Bedford & Three Bridges, and vice versa, shown with "Q" paths do run. However, a different schedule can be substituted due to overnight engineering - perhaps via Selhurst/Streatham instead of Norwood Jn/Crystal Palace, to give one of several examples. This means it is sensible to have a "Q" path which is never activated and consequently never cancelled, and another service with more accurate timings can be added and used at short notice. Calling patterns at public stops generally remain the same, though staff stops tend to get amended without much notice.
I've been using the timetable overnight Thameslink services fairly often over the last year or two (what joy...) and one tends to find they run pretty punctually and as per the public stops shown. So long as it corresponds with something advertised on a journey planner somewhere, it's usually OK to rely on. That being said, I'm not sure why some sources aren't always showing them on occasion, as they normally do show services in the regular public timetable (or the amended versions thereof), even if they are given "Q" paths.
