Yes that’s a bug which I’ve had on the back of my mind for a while. I opened an issue here: https://github.com/AnalyticalGraphicsInc/cesium/issues/5624.
Unless you absolutely need to group points together, it is still possible to use a batch table and not run into this issue. If BATCH_ID and BATCH_LENGTH are omitted from the feature table the batch table will store properties for all points. The pointSize styling will work.