I have a page with an iframe on it, and the iframe contains a form. Outsie my iframe i have more forms.
The input elements inside the iframe are tabindex 1, 2, 3, 4,
To answer part of your question, the body element inside the iframe is a focusable element.
You can prevent this effect by setting tabindex to -1 on the body of the loaded frame. For example:
outer.html:
inner.html: