Conditions under which a JWT client token expires

In order to use Sign in with Apple, I issued a JWT client according to the instructions and was able to connect without any problems, but suddenly an INVALID_CLIENT error started to occur.

The error was resolved by re-obtaining the JWT client token and resetting it.

The validity period of the JWT client token is 6 months and it has not expired yet, but I would like to know why I am getting an INVALID_CLIENT error.