When running Spotify on Windows 10 it correctly shows my two Naim devices and can playback on either with no problem.
However, if I try it from my iPhone or my iPad the devices do not appear on the list UNLESS the PC is already casting to the device. As soon as that stops they disappear from the list.
Note that this has worked but nothing has changed with my network in the interim and there has been no new software on the Naim devices, just new versions of the Spotify app . . .