Blend shapes aren’t currently supported in PolySpatial/MR mode. If you’re interested in seeing support for them in the future, I’d suggest submitting that to our road map.
Aside from that, if you have a skinned mesh without blend shapes that isn’t optimized and still displays this message/doesn’t render correctly, I’d suggest submitting a bug report with a repro case and letting us know the incident number (IN-#####) so that we can look into it.