One culprit has to do with Rockstar Cloud Services: Rockstar does not suggest playing GTA Online when Cloud Services are inaccessible. When the Cloud is down, players have the opportunity to continue playing with a temporary character, albeit without the ability to save. In this scenario players cannot access their saved characters, and any progress made with the temporary character will not be retained.
Rockstar says another problem has to do with the "Retry" button. When prompted to retry to enter a GTA Online match when the Cloud is down, some players have had all of their data wiped. Rockstar says reverting back to Grand Theft Auto 5's single-player campaign, then jumping back into GTA Online from there, should yield better results.
Rockstar says it's "in the process of determining the options for addressing the issues" and will continue to post updates.