It's the same thing on PC, you can mess with the Display settings to get it to show, but it's not really a proper solution. Needs to get fixed on Renshuu's end, I assume.
At least on PC, the problem is with the div for the main body of the pop-up box. I checked it client-side and you can fix it with CCS, but it needs a fix server-side. Not sure about the App
Edit: The div (id=ttip, class=innerbody) does have a vertical scrollbar, but it doesn't properly kick in at certain "zoom" levels.
For example, at 100% (Chrome) zoom I can't see half of the alternate grammar for 忘れない. The div overflows at the bottom and the scroll doesn't kick in. To see all of them I either have to zoom out to 80% or zoom in to 125% (scroll shows up).
The logic probably needs a few adjustments.
Edit: Alright, basically the problem seems to be that '#ttip' calculates it's max-height before "Show alternate grammar" expands and doesn't recalculate to adjust for the additional height or trigger scroll. For a quick fix you can run JS code directly from the onclick event (onclick="$(this).hide().next().show()") and recalculate the max-height that way. Needs to be wrapped in a setTimeout that defers it until the DOM updates.
You can find a better solution. If I got anything wrong, my bad