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Hanzebenger I think you should even see 10 MHz PWM - as long as it's full on/off PWM. As you can see in my pictures in which I compared battery vs non-battery waves, a few posts above: https://ledstrain.org/d/312-homemade-oscilloscope-to-detect-pwm-diy-guide/52
The wave will not look like rectangular PWM anymore, and it will float above zero, but you can see spikes that have the correct frequency.
When the flicker is just a small ripple riding on a DC signal though, as seen in "flicker-free" displays, it might not be displayed. But such small flicker may matter: FRC/temporal dithering, unresolved software eye strain - all those have just a very tiny flicker. So I imagine if a backlight has a similar small flicker, it might trigger eye strain, too.
You can see such small waves a little easier when you switch your PicoScope oscilloscope to "AC" mode. The Hantek 6022BE doesn't have such a mode.
In fact I think there is a higher frequency riding on top of your waves (where there is no thin line but distortion) which you could make visible by adjusting the time axis.
I believe the largest part of your rise times is related to the slow CCFL response. You should easily detect 1000 Hz PWM.