@MelRootsNWrites He is listed as missing on Sept 8, 1915 at La Harazée (Vienne-le-Château), with no official notice. Then it says that he was taken prisoner in Argonne (where the fighting was) and interned at the Vahn (Wahn) camp. I understand that in the heat of the fighting, he was taken prisoner, and the official authorities initially reported him missing before being officially informed.
Here's more information about this camp https://histoiresdepoilus.boitasite.com/camps/camp-wahn.php