Emailed Amazon through the kindle app with the details, though I imagine it's already been reported elsewhere and it's just them playing catch-up with sudden iPadOS 14 update.
edit:was just reading something for the last hour or so, and in the last 30 mins the flickering stopped. Hard to tell what changed, but only major changes for my part were: playing music via Bluetooth in the background, and screen temperature changing due to True Tone (I turned a lamp on in my room). Regardless, not going to pay it any more attention for now, likely we'll get either a Kindle or minor iPad update in the not too distant future.