On some version of the packages, the following options were setup:

RuntimeDirectory=mysqld
#RuntimeDirectoryPreserve=yes

This is a mistake, RunTimeDirectory shouldn't be enabled without RuntimeDirectoryPreserve=yes, and that is why this patch was enabled as a workaround until it works on buster:

746f56e57adf35a3 and followup 54969f1e7e6b70

However, the original patch for the unit file was merged, too. It should be non-problematic for single-instance hosts, but in the case of multi-instance ones, if one server is stopped or restarted, the sockets for the others will be lost in a limb, failing to be found on the visible filesystem, alerting, etc. Nothing catastrofic, but messy.

We need to patch the systemd unit on all multi-instance host so all references are removed until it is preserved on restart/stop, available on buster.