Hi Min-Hung Chou,
Are you seeing this in builder or after launched?
From your screenshot it looks like you are seeing this in builder. And based on my assumption, your page may be zoomed in a little bit and part of page is hide. If so I am afraid it is as expected. (Please correct if I am wrong. Below explanation is based on the assumption.)
What the share popup does is that it will first validate the distance between the share button and the page's right edge. If the distance is enough for the popup, it will open at right side. Otherwise, it will open left side. Note that it does not measure to the working window edge but page.
In the case, though if the page is partially hidden, the distance/ space from share button to page edge is enough to display a popup, it opens to that orientation, and it looks like cut same as your page.
What I suggest is that you can fit the page width to the current window, so you can see everything.