User works in MicroStation with pattern file acadiso.pat from Autodesk and had added several pattern definitions. When then saved from DGN to DWG file format, these added patterns disappeared.
To distinguish the added patterns from the default delivered in the *.PAT file, user had added a double underline ( __ ) in front of the pattern name. When translated to DWG-format these patterns were ommited. In order to prevent this, hatch pattern definitions need to follow these rules:Each line in a pattern definition can contain up to 80 characters. You can include letters, numbers, and the special characters underline ( _ ), hyphen ( - ), and dollar sign ( $ ).However, you must begin a pattern definition with a letter or number, not a special character.
Overview of Hatch Pattern Definitions (@ Autodesk.com)