Imported 20 .asc files and each came in as GDA94 / MGA zone 55 (EPSG:28355). Created a map containing all set to the same projection. Merged all 20 (dialogue showed the same projection)
Need all the details. Did you import into 8 or into 9? .asc files do not provide projection info, so how was it the initial projection was assigned? Were there any side car files such as world files, .prj files, etc? When you created a map, how did you do that and what was the projection used by the map? When all were "set to the same projection" how did you do that and what projection did you use? How did you merge all 20, what projection did you use, etc. When you exported, what sidecar files were created? etc, etc. This is all a matter of details, so to understand any inconsistency you have to drill into each and every detail.