@kingosticks I can confirm that installing gst-libav on Arch or gstreamer1.0-libav on Ubuntu fixes the problem, thanks! Not only for mp3, I’ve tested it with ogg and flac as well and the problem is solved.
So is the issue with the presence of the fakesink that breaks the pipeline when encoding is involved, or with internal vs. external encoder that makes the pipeline out of sync? I’m not sure yet whether this could be a regression on GStreamer or it’s just the mopidy fakesink that broke the encoding on the latest version.
p.s. Interestingly, the only physical system I’ve tested where the encoding was always working was a Raspbian that already had gstreamer1.0-libav installed.