I actually think that this “black screen” issue is unrelated to running in an iFrame. I think it is actually another instance of this error:
To verify this, I set up a version of the sample web app using web SDK 1.8.5 on an independent site and had one of the users for whom the black screen issue is occurring try to join a meeting with no participants in it. When they pressed the “Video” button they received the same black screen and all of the controls disappeared.
Inspection of the DOM shows that what is going on is that the child node below the element with id="zmmtg-root" is actually being removed (along with all its descendants). This is why the whole Zoom window goes blank and probably also explains why some part of the SDK is failing to get the width of one of the DOM nodes.