Hey @Juan de Urtubey!
Thanks for reaching out. This is usually caused by one of two things:
First, check if your organization has MFA enforcement turned on (Organization Settings > Security). When an org enforces MFA, it picks the method for everyone, and it defaults to email. If that's the case, your personal mobile authenticator preference gets overridden. You'd need an org admin to either switch the org-wide method to "Mobile Authenticator" or turn off org-level enforcement so your personal preference is used instead.
Second, double-check your personal MFA settings (Personal Settings > Security). You need to make sure the "Authentication method" dropdown is actually set to "Mobile Authenticator", not just that you scanned the QR code. The full steps are: set 2FA to Enabled, pick "Mobile Authenticator" from the dropdown, scan the QR code, enter the code from your app, and hit Save.
Most likely it's the first one, the org-level setting overriding your personal choice. Let me know if this clears it up. Thanks!