... Doesn't seem to be limited to just NM28; when using the VORAPIS V3 userscript (in KMB[Cr87]+ViolentmonkeyBETA-2.25.1 here ), the loop playlist button can be enabled (highlighted), but, sadly, does not have the desired effect ; playback of the playlist ends with its last item (what happens next depends on how you have configured things...) ...
The current YT GUI is a PITA to load and operate here in this 2008 Vista SP2 32-bit laptop (Core2Duo+iGPU+RAM=3GB) but, out of (perverse ) curiosity, I wanted to test this ; I used NHTPG's playlist posted previously in this thread:
https://www.youtube.com/watch?v=jiW7NbkhbJ0&list=PL2ZucxOJLzfVRTHxcWt5TjQ2ZCRNjc6TM&index=1
(FYI, out of the 321 playlist items, only 295 are accessible in my location - rights issues...)
The loop and shuffle playback buttons are behind a playlist "header", so not immediately visible; you seem to be able to select a "once more" playback of chosen playlist:
or a continuous loop playback mode...
In either case, when you reach the end of the playlist (NB: Autoplay must be enabled),
you are presented with a (misleading) in-player dialogue pertaining to a YT suggested clip (not contained in the original playlist),
but if you leave it be, the page auto-reloads and playback begins anew with the first item (index) of the list...