There are at least three ways to solve your problem:

1. Use fcc_clear=yes to not encrypt the copy in your sent folder

2. Use GPG's encrypt-to in your GPG configuration: https://www.gnupg.org/gph/en/manual/r1261.html

3. Add --encrypt-to to the GPG commands mutt uses: https://dev.mutt.org/trac/wiki/MuttGuide/UseGPG#TheCodeSourc...