Enter Key Not Working in Mac Mail - Big Sur

I recently upgraded to a new Mac Pro tower, as well as upgrading to Big Sur OS. On my previous Mac Pro (Late 2013) and Catalina, and every OS before that, in Mac Mail when I was composing a new email, I could use the "Return" key to move the cursor down to the next line (To:, CC:, Bcc:, Subject) and enter information into each field. Now when I hit "Return" I get a cute chirp noise and the a blue "+" appears on the right of the "To" field.

I've tried different combinations (Command + Return, Option + Return) to see if there's a new shortcut I'm not aware of, but nothing seems to work.

Anyone else have a similar problem and have a solution?

Apologies if this question has been asked and answered, I would be thankful if someone would point me to the right thread.

Current machine:

Mac Pro (2019) Big Sur 11.1

Thanks.