I figured it out. Even though I had edited nothing, something on my SD card must have been corrupted. Somebody had a similar but different issue over here: Synth doesn't play sounds from all engines
I followed the instruction to delete the workspace folder and the globalsettings file and rebooted and now it works!
Very strange. I guess first thing I’ll do is buy a higher quality SD card, JUST in case. And also back the thing up regularly. 