I have also run into that issue many times, Popup maker support group and I have been exchanging emails for quite some time and nothing for now..
I am been trying other workarounds to make the entire popup to be fixed when entering in the input field, But I see no changes in Apple devices like iPad or iPhone. Other tablet, it works perfectly.
I hope it can addressed and have some solution in the next version of popup maker.
Regards,
Krist