Edit: these suggestions are last resort type stuff tbf, hope the guys in the other thread are more help. Looks like someone suggested session restore w/ kde which makes alot of sense.

Ok that's increadibly weird. Here's some places I'd look.

I'd start looking in environment files such as ~/.bash_profile, .~/.profile, /etc/environment, /etc/profile and a few others. Maybe there's a call to the application in one of these files?

Secondly, I'd attempt to write a bash script to walk a directory tree, cat out files, pipe it through grep and get every instance where VirtualBox is mentioned in a file. Trying the name of proccess, or of the executable too.

I have a snippet that may help, by replacing that bash script:
grep -Rinw '~/path/to/start/' -e 'VirtualBoxOrSmthngElse'

all credit to this answer on SO:
https://stackoverflow.com/a/16957078/11534230

Head there to see how to try and wittle down the matches. I'd start in a etc, ignore binary files with grep, and try everywere systematically

This is likely overkill lol. If you're on xorg maybe there's something in the file xorg uses for init? Can't remember the name personally but I used it to start up some processes before on system boot quite a while ago