Rockstar rolled out a new game launcher for PC today, which came with a free copy of the great Grand Theft Auto: San Andreas—and, shortly after people got their hands on it—reports that it also changed the Steam version of Grand Theft Auto 5 to disable offline mode. Rockstar has confirmed that this did in fact occur, but said that it's a bug, not an intentional modification, and that no changes were made to the GTA5 DRM.

The Rockstar support site says the studio is aware of an issue "that is affecting some players' ability to play in Offline Mode," and is looking into it. Unfortunately, that's all it says right now. I've reached out to Rockstar to see if a timeframe is available, and I'll update if I receive a reply.