How to remove proxy address from an azure ad user?

Question

How to remove a proxy address from azure AD user?
Generally I have logged to portal using personal ms account, signed up for azure and created a separate tenant, then an internal user with global admin privileges that I intended to use daily instead of the ms account.
During some experimentation related to transferring my subscription to new account, I have removed the ms account from my directory and, unfortunately, set internal account's email address to the one of the ms account.
Now I cannot add the guest user back even after changing email field of internal user.
The issue is that it says that the user with that email address already exists. The email is shown in the proxy addresses field, however I am unable to remove this address from the portal.
How to remove the proxy address so that I can add my ms account back?
Note I do not have any synced on prem AD, any managed ADDC nor any O365 subscription on this tenant, so none of the proposed solutions, like using exchange online powershell, works.
The only thing that would definitely work is deleting and readding this internal user or doing some other workarounds related to microsoft account's mail aliases, however I would prefer to avoid these.