I'm back on the Windows laptop, in touchscreen tablet mode, and comparing it directly to the iPad.
When you tap the reply box on the iPad, the popup keyboard takes up half the screen, which I suspect triggers the automatic hiding of the top blue header bar.Then if you do anything other than straightforward typing, that header bar unhides and the reply box inconveniently moves.
Whereas on the slight bigger laptop tablet screen, the pop up keyboard takes up less than half the screen, so the blue header bar remains throughout.
I've attached images of the same process taken on the two devicesView attachment 56842 View attachment 56844
I think you are describing a functionality glitch on the iPad.
Worth mentioning to Serena although I doubt if anything can be done about it especially in the short term.