If they use a direct reversal emulsion, then this is typical behavior. We called it the fried egg effect. It is due to re-reversal of the emulsion and formation of a negative image. It is not solarization.
Very nice photo, in spite of (or perhaps because of) the effect.
Thank you for this interesting explanation, Photo Engineer! You may be interested in the experience and example by Akki14 in the comments to the other picture, too.