What's the issue?

amp-iframe does not load sometimes

How do we reproduce the issue?

1. Go to https://www.upnest.com/re/buysell3
2. Hit refresh until you see a blank white box instead of the calculator iframe

I made a video demonstrating the issue:

https://drive.google.com/file/d/1g223D8PtwUqJ5RyxTKGSzDYVCBDTueUD/view?usp=sharing

What browsers are affected?

Windows 10: Chrome 83.0.4103.116, Microsoft Edge 44.18362.449.0

Which AMP version is affected?

2007040248002