In trying to reproduce the voice-loss bug, I actually produced a new bizarre bug that's never happened to me before where the volume of my Tempest dramatically decreased to a barely audible level and turning the master Volume knob didn't make any changes (or if they did they were subtle). A restart did fix the problem. Here's exactly what I did when the issue occurred, sorry if it's a bit long-winded:
1. Booted up my Tempest, it defaulted to the BurntNeon and Funk Kit project.
2. I erased all the notes in Beat 1, changed my quantization to 16th, and set Swing to 50%.
3. I went to pad B1 (the Juno Subsquare) and used the 16 Time Step view to place an Event on every 8th note.
4. Still on pad B1 I opened the LPF completely, then went to LFO1 which I set to Square, with an amount of 98, assigned to Osc 1 Frequency. I then Set LFO 1 to Sync at a 1/2 Note rate with Beat Restart. Finally I assigned the sound to Voice 1. I now had a short stabby bass sound alternating in pitch.
5. Holding down the Copy button with all the options set to 'On', I copied B1 to B2. I then Pitched all of B2's oscillators up together by holding the Oscillator Select button and shifting it up to an E, then changed its Voice to 2. I repeated this process on pads B3-B6, increasing the pitch each time until I had made a big C chord with root pitches C, E, G, C, E, G and changing the Voice so pads B1-B6 were assigned to Voices 1-6 respectively.
6. I now had a constant pattern that made use of all 6 voices simultaneously. I let the sequencer run playing this for a few minutes, starting and stopping it a few times. I then tried removing the voice assignments from Pads B1-B6 and letting it play, and then reassigning them to Voices 1-6. With the sequencer stopped and B1-B6 assigned to voices 1-6, I would play each pad to ensure that the Voices were still working. At some point during this, the audio from Tempest drastically dropped to a barely audible level. I could run the sequencer and still hear the notes playing, but they were incredibly quiet. I turned the master Volume knob up completely (I had it originally set to probably 9 o'clock) and couldn't hear a difference. I checked the Mixer section, and all the levels were set to their maximum.
This was with me just plugged in with Headphones to the Phones jack with no other cables connected. I tried unplugging/replugging my headphones multiple times with no changes... and after a Restart the Tempest's volume returned to its normal levels. Very strange. If any more detail is required that might help please let me know.