Happens on my phone, too. I suspect it’s the browser and FR’s formatting that are not synched.
I’ve been having major problems on writing any posts longer than a sentence or two on my Android. The input box covers up the post button.
On my phone it has ONLY ever been this way. You cannot see what you post. I hate freeping on the phone. Wish there was an app. I expect it on the phone, sadly. And the return button is too close to the space bar so I hit it accidentally. I don’t know how man do it on the iPhone with bigger fingers than mine.