In Reality Composer Pro I try to read it with a Primvar Reader / Geometric Property node (type float), setting the name field to AttributeTest and also to primvars:AttributeTest. In both cases the node only returns its fallback value never the actual per-vertex data. No error or warning. If I instead author the data as a vertex Color Attribute so it exports as primvars:displayColor, the Geometry Color node reads it correctly. So the data pipeline works for the color slot, but not for a custom-named primvar. I actually verified in RCP 3 too i have the same issue, but at least you can see in RCP 3 that the attribute does actually live on the mesh when selecting it. Thanks.