This is not about this thread in particular but there seems to be suggestions on several forums that this series has been left on the shelf for ages - there was one on DigitalSpy called "why the delay?" - but I don't see why it's so unusual for a series to be recorded in mid-2018 and be shown in early 2019. A lot of it seems to have come from the Partridge doc at Christmas 2017 referring to the new show coming in 2018, but I don't think it's that odd for it to have slipped back into early 2019. It still had endless publicity and a good slot, if they were so concerned about it they'd have buried it.
I went to a QI recording in February and that's not going to be on until the autumn so I don't think a summer recording for a transmission early the following year is very unusual at all.