Pages saved with auto_save.session are lost if the pages don't finish loading after trying to restore and quitting

[agus-videla]

Version info:
qutebrowser v2.4.0
Git commit:
Backend: QtWebEngine 5.15.8, based on Chromium 87.0.4280.144
Qt: 5.15.3 (compiled 5.15.2)

CPython: 3.10.3
PyQt: 5.15.6

Does the bug happen if you start with --temp-basedir?:
auto_save.session is not enabled with --temp-basedir

Description
The tabs in the session will be lost if the pages don't finish loading (or get to a certain state, i'm not sure) before closing qutebrowser again. In the next reopen the tabs will be there but without any pages to load.
I sometimes click the wrong key on my keyboard and open the browser withouth meaning too. I then close it by reflex, because I wanted to do something else, and lose wathever I was doing before.

How to reproduce
Enable auto_save.session.
Open a few tabs and load a page in each one.

Close Qutebrowser.
Open qutebrowser and quit as soon as the tabs show up.
Next reopen:

[The-Compiler]

This happens due to #5359 combined with #2176.