PWM frequency is not selected in a vacuum. The maximum is set by your transition time which is set by a bunch of other engineering decisions. It's possible that something about tandem OLEDs make them more difficult to drive faster edges on.
Do slower pixels prevent you from using a higher PWM frequency? Wouldn't that make them like a capacitor smoothing out the high frequency signal into a constant one, like in a Class-D amplifier? That would reduce strobing even more!
https://www.notebookcheck.net/The-new-Tandem-OLED-panel-in-L...