The editing page for blocks get's its list of page regions using the function get_all_block_regions() in lib/outputlib.php. This works by loading the layouts array from the current theme and recording what regions are defined.

Unfortunately, this breaks for the 'My Moodle' page. The centre region is defined in the my/index.php page not the theme so it doesn't get found.

Hence, it is now impossible to move blocks to the centre column.

I'm almost certain that this used to work in earlier versions.