Note that this issue - https://github.com/twitter/bootstrap/issues/5900 and https://github.com/twitter/bootstrap/issues/4550 - also appears in this theme.

I have tested the issue on http://tonystar.ru and it does occur on that site for iOS in portrait mode (i.e. with the button responsive menu)

The issue does not appear to have been fixed in the latest versions of Twitter Bootstrap as far as I can tell. There have been patches, but so far I have not seen them resolve the issue.