When the R banner appears, exactly the same event remains (as if it wasn’t deleted in the first place) – yes, with the same old event time too. For the Commonwealth Games event, for example, it’s set to “Sat 26 Jul 19:00”, which was probably the final transmission of the Games.
It’s not too annoying, unless pages of undeleteable series recording events end up clogging my Planned Recordings list. I’ve done dozens of such series recording events and all but 2 have been deleted – wondering if some data was wrong when the 2 events were originally set up? So it could even be the BBC to blame for this…