On iOS. Once you scroll down so far the screen dims very slightly. The amount you need to scroll to make it happen varies from post to post, but it’s usually around 2 full screens.
See linked screenshots (forgive the iCloud link).
The dimming seems to be restricted to liftoff. Brightness remains the same in settings and the top status bar is unaffected. It can be reversed by scrolling back up.
Very odd. I’ve been playing with this, It looks like the text intensity drops slightly as the bottom of the post card (or rather its shadow) leaves the screen [or maybe that’s not the trigger, more testing needed]. I’m perplexed, but will investigate. Thanks for reporting this!