This message can be safely ignored, as it is just additional data you are getting using the --debug option.
"Unknown" is probably not the right word; a better description of the event is one that PGP Command Line doesn't care about. As data is being processed by the PGP SDK, events are sent back up to PGP Command Line for possible processing (e.g. "I have made some amount of progress, so you can provide a status update to the user"; "I am about to encrypt something"; "I need a passphrase for a key"; etc.). In general, PGP Command Line will only process events it cares about. Between those two versions, we added additional debug output, including the logging of unprocessed event types.
In this particular case, the message indicates that PGP Command Line is not processing event type 25, which is a "ToBeSigned" event that is useful when you have two parties, one of which processes the data and the other actually performs the signature.
Regards,