While I don't know about how Thunderbird works in this regard, I do know that pgp/gpg encryption also verifies the sender against a known key.
This means that the cretificate you created is most probably tied to your primary email.
So you'll probably need to set up a new encryption certificate specificlly for your alias.
Yep this is the official way. There is also the quick and dirty way of adding a new ID to your existing key to enable its use with a different email sender, but be wary of the info leaks this could lead to…
Thunderbird is the leading free and open-source email, calendaring, newsfeed, and chat client with more than 20 million active monthly users across Windows, macOS, and Linux. One of Thunderbird's core principles is the use and promotion of open and decentralized standards.
This is an unofficial forum for Mozilla Thunderbird and feel free to join and discuss