I appear to have a similar issue (using Wi-Fi) with my Muso).
I don't think I have checked the buffer in the past, but the Naim app indicates that on music radio channels at least, the buffer does not go past 90%, and flicks between 70% and 90% on some stations. Interestingly, the app indicates that when I switch to Radio 4, the app indicates that the buffer fills up to 100%.
I would have assumed that the buffer is designed to perform in this way (possibly related to bandwidth), were it not for a couple of other things.
I use the Muso almost exclusively for internet radio, and when I switch the unit on, it now definitely takes quite a lot longer for the currently selected station to play, and occasionally the selected station does not play until I have toggled between channels a couple of times. I certainly don't recall this happening to me in the past.