I have had it a few times with my Orion mount (Same same) It does not seem to be a button sticking issue, rather a software one. You press a button for a slew, get near where you want to be and release the button but it keeps on slewing. A little stressful if slewing at high speed if there is any risk of crashing anything into the mount!
As you said, a quick jab on the opposite button generally stops it. It does not happen to me often as with the dual encoders on my mount I often release the clutches and point manually rather than slewing with the buttons.
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