I don't see how preloading the shifter would cause any problem and I think it's a good habit of getting into as long as you don't apply too much force to trigger the sensor.
The more important thing is we don't actually have an answer to the OP's question. Why would the bike not shift at that RPM range, anything in the manual? I doubt this problem would occur with an aftermarket sensor.
I disagree, of course. Preloading the shifter is NOT good habit. It's bad habit. You've obviously never torn into a sequential gear transmission to understand how they work.
The constant mess gear pairs slide along a bar and engage each other on the side. In order to do that, a shift fork moves them. The gears are always spinning, but the shift forks do not. Preloading the gear shift linkage causes the stationary shift fork to sit against the spinning gear pairs. If you don't think that will prematurely cause the shift forks to heat up, bend, and wear out, you're wrong. And doing that with a quickshifter in the loop can cause the switch to be tripped, interrupt the ignition, and all without the proper force on the shift linkage to begin with. In race conditions, with an engine teardown and rebuild every race or so or when crucial milliseconds are measured, it might gain you something. For all other people, preloading the shifter just causes you to post threads on here asking other people why your shit don't shift right.