Ok played around with it. The cursor sometimes gets in the wrong spot when the text box moves dynamically (like when the keyboard overlay pops up). If you try scrolling it up and down or closing and reopening the keyboard it sometimes resolves.
As to what @spudone says - my entire screen goes frozen on my phone. I quit and go back in and the same thing happens. Thatâs with any kind of reply - just reply and type in (though it wonât let me type), or quote.