There is an easy workaround/fix for this:

su - zimbra
cd /opt/zimbra/bin

EDIT zmsaslauthdctl

CHANGE:
${zimbra_home}/cyrus-sasl-2.1.21.ZIMBRA/sbin/saslauthd -a zimbra
TO:
${zimbra_home}/cyrus-sasl-2.1.21.ZIMBRA/sbin/saslauthd -r -a zimbra

(Basically add a -r to keep the domain)


Then run /opt/zimbra/bin/zmsaslauthdctl restart