Are you side loading a MOBI file to the Kindle for iOS app? If so, then don't do that. The iOS app doesn't support the newer KF8 format and will fallback to the older, less flexible format contained in the MOBI file created by the previewer. You need to instead create and side load an
AZK file.
ETA: If you still want to use the MOBI then you may be able to make it work by specifying width instead of height. I believe the older format supports that.