Is there a way to set the soft eraser as the default eraser when I turn my stylus upside down?
What is supposed to happen is that the ‘stylus-tip’ brush preset in use and the ‘eraser-tip’ brush preset in use are remembered for the next session by being stored in the kritarc configuration file.
There have been problems with various aspects of operation when switching between stylus-tip and eraser-tip. These are the subject of ongoing formal bug reports.
At the moment, with the 4.2.9 beta, it seems that the eraser-tip selects the Eraser_Circle brush preset on first use, regardless of the correctly stored name of the last used eraser-tip in kritarc.
As far as I can tell, all you can do is start krita, use your eraser-tip to select the soft eraser brush preset and it will be remembered for the rest of the session.
One of the developers may have more information about this or a better idea of what you may be able to do.