Has anyone been able to find a solution to this? We are noticing the same issue on vSphere 5.5 as well. If we perform a storage vMotion to another data store on the same SAN, it correctly updates the "Used Storage" amount. It seems to be an issue at the host level since it shows the issue when you connect directly to a host as well as through vCenter. We are using a SAN different than previous posters, but it is on the supported HCL.
To some extent, it appears that storage DRS looks at the "Used Storage" when calculating stats for moves, since I now see it failing storage vMotions periodically due to insufficient disk space. The total Free Space per data store appears to be correct regardless of this bug.
Thank You!