I have had a similar issue happen with the output volume cutting out completely, and canāt seem to find much of a pattern.
The first time it was after removing the output cables and plugging back in to another set-the audio cut out, but meters showed output. A reboot fixed it. It happened at least one other time randomly(not while I was plugging or unplugging cables), and was fixed again by a reboot. One night I was recording with a group, using Norns strictly as an output source and I had it happen numerous times, to the point that I had to stop using it. That night, I would get about 30 seconds of output and then it would cut out. Initially I thought it might have been an issue with the file I was using with Gemini, but on reboot I tried both different files in Gemini and different sound generating apps and had the same problem. Eventually it would produce no sound on reboot. Headphone out also would produce no sound. The meters consistently showed output, however.
When I started it up again a few days later, all was normal and it played for hours with no interruptions. At that time I was on the most recent firmware(there has since been an update I have not yet installed.)
That was the last time I used it, so I just booted it up to check its behavior today and, after initial startup, it would not respond to any button presses. A reboot has fixed that. Mine is an actual Norns unit, not a shield. I purchased it used and have had it for a couple months. Itās generally been reliable, these are the only issues I have experienced.