unable to add external alias

I am using a custom domain and would like to be able to create external aliases to send email using aliases on my custom domain. I used to be able to create external aliases, but I only see "Add new alias" for @mailbox.org followed by aliases with custom domain that I created previously (32 of 50). The option to "Add external alias" has been removed. I'm not trying to send email using an alternative, external sender address or a disposable email address. I'm trying to use an alias for a custom domain as described here: https://kb.mailbox.org/en/private/custom-domains/using-emails-with-a-custom-domain/#step-3-enter-your-email-address-as-an-alias-in-mailbox

On my standard plan, I'm supposed to be able to create 50 aliases for custom domains. Is this just an issue with my account or is this a system-wide problem?