I can confirm the above behaviour (also a Volumio Primo user). With 2.xxx continuous play (and gapless) over UPnP worked without a hitch, but with 3.xxx advance track has to be done manually - also elapsed time does not work (I’ve tested with mConnect, BubbleUPnP and also Foobar on a Windows PC).
I’m not a heavy user of UPnP so for me it is a minor issue.