I was guilty of not being precise enough here - one particular way of adding data works (the same kludge that icecast uses for mp3 and aac) - but in general the modern ogg way does not. RP uses the kludge, JB Radio (and many others) uses the ogg format.
The …/flacm URL for RP should work most of the time but RP have told us that the …/flac URL is more stable. The metadata in this particular case is added separately by Roon using @alec_eiffel 's metadata service.
For this particular issue, could the DNS cache for the NUC be corrupted?