I noticed after posting there was a QNAP PMS update made available yesterday 1.19.1.2621-b249be3b0 April 3, 2020 which I’ve just installed and touchwood seems to have fixed it (for now anyway).
I’ve been using Channels DVR and it’s Server backend and it’s so polished it makes Plex DVR look clunky and lacking in features. But finances being as they are currently during this Covid-19 crisis has made me drop back to Plex temporarily albeit grudgingly.