Design Manager erases the document number field of a part when using Save As.
Solution
This can occur if you have the option 'Use Document Number and Revision ID' turned on. If you set the 'File Naming Rules to 'None', then the Document Number will not be cleared.
Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: window
OS Version: n/a
Product: SOLID_EDGE
Application: DOC_MANAGEMENT
Version: V109.0
Function: SEDM
Ref: 001-7861246