I've noted sometimes after
apt install something
deb files are left in
/var/cache/apt/archives/ but sometimes they are gone right after installation.
I've read comprehensive explanation of install process in Ubuntu in Why are there deb files in /var/cache/apt/archives/? but there is nothing there on how/when
deb files get deleted from cache, it says:
.deb package files remain cached in /var/cache/apt/archives even after successful installation in case they're needed for future use.
Web search gave no results to the specific question of my concern. Why sometimes
deb files get deleted immediately after installation?
ADDED: Just before installing that
something I manually deleted all previous
debs from /var... maybe it mattered and the reason. Probably I should try to use
apt clean (just read about it in https://www.cyberciti.biz/faq/can-i-delete-var-cache-apt-archives-for-ubuntu-debian-linux/) instead of manual