Email Account "MicrosoftAccount\user@domain.com"

I'm using Office 365, and Outlook was repeatedly prompting for my Outlook.com account password. Before my email address, it had "MicrosoftAccount" (i.e., "MicrosoftAccount\user@outlook.com").

I was able to clear this by deleting all references to it in Credential Manager.

One of the apps that I use (RingCentral Phone) connects to Outlook. I keep getting a Windows Security popup requesting the password for my account. The username is showing the "MicrosoftAccount\user@outlook.com" prompt for username.

Is anyone aware of how to remove this? I've uninstalled/reinstalled RingCentral, searched through the registry, etc. without any luck.