I see the same thing on the PC server's Standard View while playing to Android BubbleUPnP on phone (no DSP) and select the phone renderer under Playing Now.
Switching between various display options sometimes displays the cover art:
RMB in the blank Content Panel > Cover Art Style > Plain (no border).
RMB in the blank Content Panel > Cover Art Style > Plain.
The cover art may or may not show up.
Switching between:
RMB in the blank Content Panel > Cover Art Scale > Stretch
RMB in the blank Content Panel > Cover Art Scale > 100%
RMB in the blank Content Panel > Cover Art Scale > Stretch
Again results vary.
Once cover art shows up it sticks as play continues through the album's tracks. Double click to change tracks and it disappears again.
On the other hand, show or hide [lyrics] works consistently, with or without cover art, per the menu item:
RMB in the blank Content Panel > Display Lyrics
(Sometimes switching Display Lyrics on/off brings back the cover art.)
I also find that the only option available to Customize the remote's tab view is "Files List". So this "view" cannot be customized like a normal Audio view (BTW, same for Playing Now, so this aspect may be normal for a renderer).
Looks like a bug.
Additional Notes: Described behavior is not limited to playing to Android BubbleUPnP. I see the same (visible track list, but cover art mostly does not show) when playing to a Chromecast (DLNA) renderer and selecting the renderer under Playing Now. When play transitions on its own from one track to next the cover art usually appears. Changing tab menu item from "List Style > Details" to "List Style > Thumbnails" is also inconsistent (nothing shows at bottom of Content Panel), while at the same time "List Style > Tiles" includes thumbnails. Cover Art seems to be read from the audio file, not my image sidecar files.
3/19/2024: multiple edits.
Win 11 Pro (64-bit) | MC 32.0.26 & .27 (in beta)