Rotating the iPad to landscape fixes it, but going back to portrait, the issue persists.
I can reproduce it here on meta, as well as my forum (v1.9.0.beta14 +19) on my iPad Air 10.3.3 and a user on an iPad Pro with the “latest” iOS reproduced it as well.
That’s an option, sure - but in this case it took 5 line returns to get the full list to become visible. You then have to delete the 5 empty lines, too.
Regarding it fixing itself in landscape orientation - I take that back. I could not get it to work properly replying here.
It happens to me using iPad Chrome Firefox Safari - portrait and landscape
What I usually do is close the keypad, tap the member name to insert it into the post, then focus the textarea to open the keypad again.
I think it might be able to be fixed with some type of position offset detection to open it either up or down depending on the circumstance, but that might be trickier to do than I imagine.