It probably has to do with other styles assigned to the textarea, maybe the ::placeholder as it hides when typing (I assume on focus)

In any case. In the screenshot the scrollbar is inside the textarea as it aligns with the resize control on its right. This is basically all the info needed to deduce the textarea overflow is the culprit.

But could be that the overflow-x is just a bandaid hiding the issue causing the overflow in the first place, like crazy styles on the placeholder.