It sounds like you're trying to summarize one or more of the fields that are automatically added by ArcGIS, such as Shape_Length, Shape_Area, etc. An in_memory feature class doesn't have these attributes. I suspect they are created when the feature class is actually witten to disk.
To summarize any of those fields, you'll need to direct the output of "multipart to singlepart" to your scratch workspace or to some other persistent geodatabase.