If you tried a factory reset and it still didn’t resolve anything, it would be time to email support@neuraldsp.com and have them log the issue and hopefully provide a quick solution. Usually the numbers changing don’t reflect the volume actually changing so you should be able to use it while NDSP resolves the issue/RMAs etc. I haven’t seen many reports of this issue but it does come up every now and then.
The number jumping on its own while the audio level stays put does look like the encoder rather than the software, especially since a factory reset changed nothing. Before you RMA, it’s worth a couple of quick checks so the email to support is airtight: does it still scroll with nothing plugged in and running on its own, and does it happen while the knob is completely untouched or only just after you brush it? A very light clean around the knob shaft (and making sure nothing on top of the unit is resting against it) has fixed a few phantom-encoder reports. If it only jumps when you touch the chassis or on a particular power outlet, note that too - grounding can make an encoder read noisily. Filming a 20-30 second clip with the unit sat still, hands off, is the single most useful thing to attach for support. As MP_Mod said, the numbers usually don’t reflect real level changes, so you can keep gigging with it in the meantime.