I remember doing similar. I would intentionally kill my browser/computer without gracefully shutting down the browser. Literally everything else I would go and shut down normally, browser needed to be crashed.
For Firefox, Session Restore came in version 2 back in 2006 - Wikipedia
I'm not sure when Internet Explorer added the feature, but they removed the automatic restore in version 8 and made it manual in 2009 - Wikipedia. I feel like Internet Explorer 6 had this feature, but I can't remember now.
Not sure when/if older browsers like Netscape Navigator had this feature.
I've learned that it first came about as an explicit feature around 2006, I was not awake as a proper human at this point.
Regarding the privacy concerns.
Just now, my friend used her google photos AI to search for photos of me and my dead cat using my name.
I've come to accept that my information is readily available and that I am but a humble fish apart of a school surrounded by sharks, playing a numbers game for survival.
That sounds more like your phone is agressively shutting apps down to save memory and energy. Some brands are known to do this more than others. You should be able to tell your phone not to close that app when on the background.
That might be something with voyager, I agree with the other person that it sounds like the device itself is closing the app due to some sort of battery setting, but if it's not that then it's something at least. I've tried a handful of different lemmy clients now currently using eternity and none of them did what you were describing there.
That or operator error(less likely) such as using the back button to go back to the home screen instead of using the home button or something simple like that