(Ubuntu 24.04.1, TigerVNC 1.13.1+dfsg-2build2)
On startup, tigervncserver (via Wrapper.pm) copies ~/.vnc/passwd (and other credential files) into /tmp/tigervnc.XXXXXX directory and tells Xtigervnc to use those instead. There are at least two problems with this:
1: On Ubuntu, automatic age-based cleaning of /tmp is enabled by default. This is problematic in general (see bug #2088268), but specifically the /tmp/tigervnc.XXXXXX directory can get removed. If /tmp has the noatime mount option, the removal always happens 30 days after the VNC server is started. Without noatime, the removal happens if there is a 30 day period without any new connections to the VNC server. When the directory is removed, the VNC server becomes inaccessible.
2: If the credential files (e.g. password) in ~/.vnc/ are changed, the running VNC server will not pick this up and will continue to use the old cached credential files.
I think there should at least be a mechanism to enable/disable this caching behavior via a configuration file (or a command line argument). Also, if such caching is done, I think the proper location would be under $XDG_RUNTIME_DIR instead of /tmp.
It sounds like tigervnc needs its own runtime directory (e.g. /run/tigervnc/) which it can control with its own tmpfiles configuration (or otherwise). It should not assume that things in /tmp will be around indefinitely.
If you want to change the default on your system, you can do so with a /etc/tmpfiles.