When files in /tmp actually disappear
Expecting /tmp to be wiped on reboot, a three-month-old file is still there. Modern distributions clean it with systemd-tmpfiles-clean.timer, configured in /usr/lib/tmpfiles.d/tmp.conf:
q /tmp 1777 root root 10d10d means ten days without access. That is atime, not mtime, and relatime mounts update atime infrequently, so deletion can come later than expected.
Clean now with systemd-tmpfiles --clean. To change the policy, drop an override in /etc/tmpfiles.d/ rather than editing /usr/lib.
Putting sockets or lock files in /tmp is a common mistake: the cleaner does not care whether you are using them.