When I open a tab file that's been generated in QGIS using EPSG 28355 (MGA 94, z55s) using QGIS 3.10 the projection is transformed to 5551
This is not just one dataset, it's been happening to any data that starts in 28355 or 28354.
It works fine for shp/gpkg etc...the issue is just with the .tab format.
As per my comment to a reply -this has been happening in all QGIS 3x versions and the test is simple.
Save it as a tab file and a shp file using save-as rather than "Make Permanent"
In a new project try and bring the data in. The SHP is fine but the TAB asks to transform.