Back button overlay iframe's content

We have a problem in safari, when we click back button after some action or simple navigation on site, <iframe /> content are not always created correctly.

For example, on page, we have installed <iframe /> tags:

<iframe src="iframe1.html">content iframe1</iframe>

<iframe src="iframe2.html">content iframe2</iframe>

After back click button we have:

<iframe src="iframe1.html">content iframe2</iframe>

<iframe src="iframe2.html">content iframe1</iframe>

Sometimes, content of our iframe move in the other iframe.

How can I fix it?

Pls help!!!

Safari Version 13.0.5 (13608.5.12).