Using Plex v1.13.5.5291 and an HDHomerun CONNECT dual tuner, I had a show set to record which seemed to disappear after the time slot.
This was the only show on Monday, at 9:00pm with 2 minute leader (Salvation s1e7), and regularly records fine. I also remember seeing listed it in the “Recordings” grid the day before, but when I checked on the recording a couple hours after it’s time slot the column was empty as if nothing was ever scheduled there. No error messages or anything I can see.
The NAS it records on was being used to watch a movie from the libraries, but no live tv viewing and nothing that should use up the tuners.