This issue was mitigated by a new version of the gpstime package which is released for Debian12 in last week.
The new version of this package was deployed to all workstations.

When leap seconds information in the system time zone file is expired, the new version of gpstime package finds an online leap seconds info. from multiple sites. Of course, same problem is reproduced when all referred sites are expired. But the risk is reduced from the old version.

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This update was applied between v0.7.0 and v0.8.0.
https://git.ligo.org/cds/software/gpstime/-/commits/master?ref_type=HEADS

For Debian12, only v0.6.2 and v0.8.2 are distributed in the stable repository for now. So we need to install v0.8.2.
controls@k1ctr4:~$ apt-cache madison gpstime
gpstime | 0.8.2-1
gpstime | 0.6.2-1