MicrosoftOnline SSO in Excel Add-in webview doesn't allow to select different account for sign-in

Copper Contributor

Good day everyone! Hope this is the right place to ask questions and get help on Excel Add-in development.
One of our users has a weird problem -
He uses two accounts on his machine, both of them use microsoftonline.com to authenticate through sso - account A and B.
Our app has a web version and an Excel add-in. To login into the app regardless of the version user needs to type his email first, then system detects whether it's a federated user and if yes, it redirects him to the configured idp (microsoftonline.com in this case), user signs in there and is redirected back to the app. Pretty standard flow.
Add-in uses WebView with our web version for that, so the behavior should be the same as the web version.
Now here's the thing - when user is redirected to idp in web (even Edge and IE) - he can choose any of his two accounts to login with (A or B). However Excel Add-in WebView doesn't allow him to choose, it selects account A instead and shows an error (User account from identity XXX does not exist in tenant...), as if account B does not exist for it at all. But user needs to be able to choose account B, similar to Edge and IE.
How can we make account B appear in microsoftonline account selection for the Excel-Addin WebView? Maybe there's a way to clear Excel WebView cookies to stop account A being selected automatically?
We have already checked "Email & accounts" system settings and he has both accounts under "Accounts used by other apps section". He is logged into Excel with Account B, but this takes no effect.
His Excel version is 2106 (build 14131.20320), his OS version is Win 10 ver 19043.1110
Here's also his user-agent if that helps - FxVersion/4.8.4390.0 OSName/Windows OSVersion/10.0.19043.0

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