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
But when I open it in Mapinfo v17 it's correctly reported as 28355
ogrinfo thinks that it's EPSG:9122 and 9001
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.