I’ve found the bug. Try this fix and let me know whether it solves the issue before I open a PR.
sed -i.ori 's/"\$WEBMAIL")/"WEBMAIL")/' /usr/local/hestia/bin/v-rebuild-mail-domain
for user in $(/usr/local/hestia/bin/v-list-users plain | cut -f1); do if [[ "$("/usr/local/hestia/bin/v-list-user" "$user" json | jq -r '.[]|.U_MAIL_DOMAINS')" -ge 1 ]]; then echo "${B}[ + ]${N} Rebuilding mail domains for user $user"; /usr/local/hestia/bin/v-rebuild-mail-domains "$user";fi;done