Thanks! I was also looking for a way to force 90 hz on some apps and I didn't know there was an option to force it.I just found a solution for this. I had this issue and it got resolved when i turned on "Force Peak Refresh Rate" setting from Developer Options. It caused an increase in battery drain though. So this might help you too.
I think the flickering issue is fixed.I'll let you know if it shows up again.