This is resolved by upgrading to Service Pack 22, and it is included in the release notes. The cq-msm-lockable property is applied to cq:robotsTag instead of cq:robotsTags where the property is stored. You can view this in CRXde at:/libs/wcm/foundation/components/basicpage/v1/basicpage/tabs/advanced...

Was this issue resolved? I'm having the same issue. Thanks.