To recreate (iOS 10, 11 & 12: Safari & other WebView browsers like Chrome):

1. Place focus on the 1st page input
2. Place focus on the 2nd page input
3. Place focus on the 1st iframe input
4. Now try to place the focus back on the 2nd page input (typically I cannot)
5. Place focus on the 1st page input
6. Place focus on the 2nd page input (this should work now)
7. Now try to place focus on the 1st iframe input (typically I cannot)

Once the focus can't be placed in the iframe, the only way to allow editing of the 1st iframe input is to put the focus on the select in the iframe. After it has focus, I can then put the focus on the input again (only to lose it again if I go up through the 1st & 2nd page inputs).

After you're done, try the work-around

Spacing for scroll