This issue may be a bug. When 'write permissions' are applied to a project the grantee has permissions to write to (ie. update metadata) all existing datasets within the project. However, if a dataset is added to the project after the grant occurred by a user other than the grantee (such as rootAdmin or another write privileged user) that dataset will not be write-able by the original grantee. It should be.

This issue has been described by the ACADIS Curators as "permissions not sticking".