Thunderbird erases first part of email address when "@" is typed

When I attempt to type in an email address in the "To:" field in Thunderbird, the program erases the first part of the email address when "@" is typed. For instance, if I want to type in "[email protected]", I write "person" and when I then press "@" I experience that "person" gets deleted.

I do not recall seeing that behavior before and I do not think I have changed any configurations. The Thunderbird version is 68.2.2.

I can write "@example.org" HOME "person" to get "[email protected]".