I could not reproduce that problem on my iPad either. I would suggest removing filemaker from the problem device and reinstalling. I sent some strange problem on the iPad and iPhone with FMGo. It a good program and has improved a bunch but it still has bugs. I reported an issue I was having that occur on both my iPad 3 and iPhone 5 and TSGAL said she couldn't reproduce on her iPhone 5s or iPhone 6. I know that some keyboard issue are different on the iPhone and iPad, and it may be so with different version of iPads. I would test a different number keyboard and see if that works.
Easy Fix, Just add space to the bottom of the layout in layout mode. That way the keyboard will not cover the content.
Also, if want to go farther, add another layout just for iPad or iPhone and Desktop.
Check out the Link Below
The keyboard takes up half the screen. You can't waste space on an iPad, iPad mini or iPhone, it is not like it has a 20inch monitor, so leaving half the screen blank for the keyboard is not a feasible solution. The field should slide up above the keyboard, and the testing I did on my iPad the field did slide up, so there has to be some issue causing it to fail on his client device.