Consider:

 auto& userPartition { qspi.userPartition() }; // This returns a reference to a block device.
 LittleFileSystem2 userFilesystem( "user" );
 auto err = userFilesystem.reformat( &userPartition );

This crashes (depending on where it is in the code base) because reformat calls unmount. Unmount checks whether userFilesystem._bd is null and if not tries to deinit() it. Sadly the constructor for LittleFileSystem2 does not initialise _bd so it's whatever is lying around. LittleFileSystem doesn't have this issue as it initialises its fields properly.

I know this is a dead ARM port that the core just uses, but I spent an hour tracking it down so I thought I'd log it for others to find.