Yeah I noticed that to I couldn't reproduce it while running on my desktop, but on the device It seems to happen. it's a combination of moving from the "0" state with the slider or with the buttons.

They fire dependently, the buttons update the slider and slider updates the thumbnail of the image. For some reason there are times where I don't get a signal for example
on_QSlider_valueChanged might not get signaled, but the slider has changed.

there's also on_QSlider_Moved which I don't trigger on that may recieve the signal. Something To look into, I'm irritated to know It's not just me.