This seems to be a problem that showed up for the first time a year or so ago. Someone say if I'm wrong. Here's my take on it. There needs to be a relationship between the force applied to the adjuster and the strength (seated pressure) of the valve spring. That relationship would have to be that the valve spring pressure would have to be greater than the force applied by the adjuster. If that were not true the valves would do what yours are doing. Now if the valve springs were failing I think it's all but impossible to all fail at once. I have seen on cars where a rare weak spring would cause a miss. But never all. So my thoughts are this. The adjuster must be creating too much force for some reason. I have noticed this only seems to rear it's ugly head on cold starts especially in cool or cold weather. I'm guessing that the cold thick oil spikes pressures and increases the force applied by the adjuster. I have never had an issue as most have not. I always use 10/40 car oil. I don't start mine below 40 degrees as I can't ride it anyway. I think you have all the stars in line and it is really haunting you. Do you run a heavier oil? Diesel oil? I agree with Driverider that the relief valve might come in to play but even if it is working properly it is possible it might not react fast enough or with enough volume released to avoid a spike. Once that oil is pushed in the adjuster under high pressure it's kind of tricky to get it bled off. Some have had luck turning the engine backward and in theory it should work over time. If I were you I would think about thinner oil the 10/XX oils etc. Also if it's cold when you start it some kind of heat to warm the oil before you hit the start button. Heated dipstick comes to mind. I know it sounds like an over reaction as no other does it but if you have/keep the heavy oil it might be a best solution. It's just my opinion for what it's worth as they say.