Right, okay - so there's two things going on.
The mobile device should not be automatically capitalizing the first letter, unless the mobile device is for some reason ignoring the autocapitalize attribute we're setting on the input.
The second thing that's going on is that IdP usernames are case sensitive. I think that depends on your backing user source, actually? I'm going to tag @jspecht in, because this isn't my area of expertise, but there's some context in a thread here that alludes to some of this being intentional (but this is a long time ago now, so I'm not sure if anything's changed):