I am copying my reply from the support ticket you opened in this thread instead anyone else is interested. You can reply in the ticket in case you have any further comments.
This is the default behavior of scrollable elements in a hybrid app/mobile browser. You can see that the same behavior will be observable if you create a scrollable div and put enough content to make it overflow.
A way to modify this behavior is to set the following style rule to the scrollable element:
This way, a scrollbar is visible while the user is scrolling the textarea.
I hope this helps.