some suggestions:
Make sure you're using private/incognito browsing.
Start by logging into your new tenant with an account 'owned' by that tenant.
For example: if you're tenant is contoso.onmicrosoft.com, login with bob@Company portal .onmicrosoft.com.
In Azure AD, add your personal microsoft account as a guest account:
Users > +New Users > Invite External User
Hope that helps.
sorry, portal.azure.com and azure.microsoft.com were both tried; same results