OMG! Thank you Daniel! This has been driving me nuts in both Telegram and OneNote. I'd stopped using forum apps altogether. This is the exact fix. I disabled the Grammarly setting and everything works now. I also decided to switch to the Microsoft SwiftKey keyboard instead of the crap Samsung keyboard.
Comments on Buggy Samsung–Grammarly keyboard update puts profit before typing
I don't have a Samsung but I've used the Grammarly keyboard for a few months. The official one. I've had similar scroll-jumping issues in couple of apps. Will switch to the Google one for a while and see if the random scrolling goes away. I think it mostly a problem with "web" apps. I also didn't think the problem could be the keyboard app, but it makes sense when I think more about it. Android 11.
Crap! crap! crap! I traded my Galaxy S20 for an iPhone last month because I couldn't live with the random text replacements. I write a lot of long work emails from my phone and this was a complete deal-breaker for me.
Large chunks of texts would just mysteriously disappear when I wasn't looking. Paragraphs kept swapping order. Incomplete sentenced (that I KNEW I'd compelted earlier) scattered throughout my messages. It was a real pain in the apple pie. Completely unusable.
It didn't occur to me that it was the keyboard! I didn't know I could install other keyboards either, so knowing that alone wouldn't have helped. (I could never have typed anything this long with the S20.)