Report embeded in IFRAME doesn't load completely

I have a bizarre situation where I have two Power BI reports and both are embedded into an IFRAME and separated by Tabs (Bootstrap).

I could see the first have loaded completely but if I switch to the next tab I could see the reports' first few visuals are visible but others are not shown or loaded; It's a blank area that shows. However, if I click somewhere on the blank area, the rest of the visuals will appear. Sometimes, both of the reports show the same behavior, however, this doesn't happen often.

I took a video out of it: https://youtu.be/LmKNQ3FKZis

What can be the problem?