Trainers, Niantic will be releasing a new update soon and as confirmed, players could be logged out and asked to sign in back again.
As confirmed by Niantic Support on their official Twitter account, the next update could cause players to be logged out of the app, which means that you have to sign in back again using your login email.
The next update to our servers could cause you to be logged out of your Pokémon GO or Ingress accounts. Please double-check your login credentials ahead of time to ensure continued access to your account. You can view your login email under the account section in Settings.
— Niantic Support (@NianticHelp) June 17, 2019
This might affect Adventures Sync too, so make sure to check you’re logged in after the update. If not, go to Settings to login and check if the feature is working.
More info to follow, so stay tuned!
How can I check if I can still use my email
Thanks for the comment. You can always check your login email under the account section in Settings.