Hi,
this is a known issue and will be fixed in the next version.
The reason for the current behavior is that iOS does not handle user-zooming via pinch in the expected way. This means that if a user does a zoom via pinch on the embedding page and then taps the 'expand gallery' button, the gallery will not be displayed with the correct layout. To avoid this issue, we decided to open expanded galleries in a new URL (on iOS).
There are a couple of disadvantages of this approach, so in the next version we are going to give the option to expand in the same page or in a new page. To allow expanding in the same page, user zooming must be disabled in the embedding page.