One explanation is that Janus could be undergoing a rare transition that has been predicted to occur during white dwarf evolution.
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The star’s intense gravitational field causes heavier elements to sink to the core and the lighter elements to float, creating a two-tier atmosphere of helium below, topped with a thin layer of hydrogen (the lightest element). When the star cools below about 30,000C (54,032F), the thicker helium layer begins to bubble, causing the outer hydrogen layer to get mixed in, dilute and disappear from view.
And apparently if the star's magnetic field is asymmetrical, that might cause the transition to happen lopsided like this.
