Mac OSX Yosemite 10.10.4
MacBook Pro (Retina, 15-inch, Late 2013)
Processor 2.6 GHz Quad Intel Core i7
Memory 16 GB 1600MHz DDR3
Details to recreate issue:
Open Mediabay and under the “Attributes Inspector” click the plus sign to “add user attribute” and you will notice that it does stay in the inspector like everything else.