I encountered the same problem, it's a limitation of the 2012 version, here is the answer my reseller gave me
The data retrieved from the welded parts list is the custom properties. When inserting a part into another part, only the custom properties are carried over.
Starting with Solidworks 2013, the "Configuration-Specific" properties are also carried over.
To get around the problem, you can: - use as many rooms as possible without using the configurations - Create an assembly - manually enter the plan number
By the way, Solidworks does what we ask of it: it retrieves custom properties and not specific properties.