The quintessential example is "cheval" (horse) which becomes "chevaux" (horses), which is the rule they're following (or being cute about). Un mistral, des mistraux. Un ministral, des ministraux.
(Ironically the plural of the Mistral wind in the Larousse dictionnary would technically be Mistrals[1][2], however weird that sounds to my french ears and to the people who wrote that article perhaps!)
[1] https://www.larousse.fr/dictionnaires/francais/mistral_mistr... [2] https://fr.wiktionary.org/wiki/mistral
the classical way to pluralize "–al" words:
un animal → des animaux [en: animal(s)]
un journal → des journaux [en: journal(s)]
with some exceptions: un carnaval → des carnavals [en: carnival(s)]
un festival → des festivals [en: festival(s)]
un idéal → des idéals (OR des idéaux) [en: ideal(s)]
un val → des vals (OR des vaux) [en: valley(s)]
There is no logic there (as many things in french), it's up to Mistral to choose how the plural can beEDIT: Format + better examples
The "Trésor de la langue française informatisé" (which hasn't been updated since 1994) says val is deprecated, but it's common in classic literary novels, together with un vallon, a near synonym.