It can take up to 24 hours for the signal to be properly calibrated.
When I compare with various "internet clocks", mine is accurate to the second.
From Wikipedia: For watches powered by larger cells, reception is common every full hour, for wristwatches with button cells or with solar cells only once a day, usually between 1:00 and 4:00 in the morning.

If there is reception and the clock is set correctly, it is accurate to the second. If this is the case and it still deviates, it would be defective. This is rather unlikely, but not impossible, especially if it is already old.

Unfortunately, the link to the instructions did not come complete.
Here it is complete:
https://support.casio.com/de...

I don't think the clock is broken.
Maybe it's in a dead zone, or the reception is disturbed by a mobile or a computer nearby.
Or maybe a setting is wrong.
The instructions can be downloaded here:
https://support.casio.com/de...