Enable Persistent Login in Azure B2C: Ensure that persistent sessions are enabled in Azure AD B2C so that the login session is retained and shared.
Use the Same Login Domain: When opening the Canvas App in the web view, ensure it uses the same authentication context and domain as the token issued for the custom app.
Configure Power Apps to Use Azure AD B2C: Set up the Canvas App with Azure AD B2C authentication in the Power Apps environment. Ensure the app's authentication redirects match the Azure AD B2C tenant.
pls try and let me know if you need more details.
thanks.