Done some additional troubleshooting:

The UTC time on this HWID 1091 device shows correct. It's just the local time that is 1 hour behind. One possible cause is the device does not consider Day Light Saving correctly. We'll know that first weekend of November when the clock moves 1 hour back. If the nüvi then still lags by one hour, we know the time database is correct *), but for some reason it has an offset of -1 hour.

*) Actually we already know that the time database is correct. Otherwise all my other devices would demonstrate the same issue.

So the other option is that the FW or internal settings somehow got damaged. I know I already reloaded the same FW several times now, but a very wise teacher at this forum once upon a time taught me that rolling back FW and then reinstalling the new FW again does more "deep cleaning" than reverting back to factory settings.

I therefore flashed FW version nuvi2x5W_GCDFile-MTK__760.gcd into the device via WebUpdater. After reboot however the same 1 hour lag still shows.
I then let WebUpdater load the latest FW version nuvi2x5W_GCDFile-MTK__780.gcd into the device again. Still no luck.

Not sure if there is anything else I have not tried yet. Running out of ideas here.