I have the same kind of problem with "0" and "9" but it happened to me before with a synth of a different manufacturer (I can't remember which) and I know that most of the times it gets fixed by clicking it fast 30-40 times. It's not a huge problem for me because I don't use the synth on stage and I don't mind repeating the procedure every 4-5 days, but It's a shame for that kind of synth.
Yes, I've had synths from other manufacturers with the same problem. Blame it on poor quality TACT type switches. Those should have been banned years ago. They're not reliable, not comfortable to use, and feel "cheap" anyway. Any kind of mechanical switch that uses the metallic dome convex/concave method of making momentary contact are bound to develop electrical contact problems over time, either by oxidation/dirt accumulating on the surfaces, despite having very optimistic MTBF times from the manufacturers of such switches (supposedly millions of times trouble-free operation in some cases). I've yet to see that in practice from ANY such devices.
Have I mentioned that I hate TACT switches?
