I'm running 1Password 7 beta on windows using both a personal 1Password account with a couple of vaults in and a shared vault in Dropbox for work.
I've noticed that the "Vault for saving" setting resets to saving to Dropbox whenever I lock and unlock 1Password again.
1Password Version: 7.1.567
Extension Version: Not Provided
OS Version: Windows 10 1803
Sync Type: 1Password account & Dropbox