Is it a specification that a request is executed to the server when the save button of the browser's

Hi,This is my first post.
Let me ask you a question.

I am displaying a PDF on a page using an iframe.
The browser viewer is displayed when the page is displayed.

It is natural that a request to get a PDF will be executed at the time of initial display,
The same request will be executed when the save button of Viewer is pressed.

For HTTPS communication, occurs in 89.0.4389.82.
For HTTP communication, the phenomenon occurs when updating from 89.0.4389.82 to 89.0.4389.90.

The same phenomenon occurs in Microsoft Edge(Chromium).
Does not occur in IE or Firefox.

Is this a specification?