GnuPG 2.1.x now correctly finds User IDs that contain a raw e-mail address like
'foo@example.org' when the user requests '<foo@example.org>'. That's good!

However, it appears to only work for those keys stored in pubring.kbx, and does
not work for keys stored in pubring.gpg.

You can recreate this problem with a fresh $GNUPGHOME with the reproducer below:

export GNUPGHOME=$(mktemp -d)
gpg --quick-gen-key --batch --passphrase '' test@example.org
gpg --list-keys '<test@example.org>'
gpg --export '<test@example.org>' > $GNUPGHOME/pubring.gpg
mv $GNUPGHOME/pubring.kbx{,.bak}
gpg --list-keys '<test@example.org>'

not found above, but it is seen below:

gpg --list-keys