I use toolbox parts routinely in my assemblies and assign my company's part number as a custom property in the property tables. On multiple occasions the toolbox appears to have lost the part number for no apparent reason. I know that the toolbox part was saved with the custom property, and I know that in most cases I had opened that assembly (and assembly drawing) after making the change and the change was still. At some later time, when the assembly was opened, the custom property (part number) was no longer there. I am using SolidWorks 2017 and the toolbox is on my hard drive, I do not use a PDM (I'm the only user).