Mafia II Definitive Edition does not support HDR on PC at all.
Talk:List of games that support high dynamic range display (HDR)
Mafia II Definitive Edition
> For the first time ever, Mafia II: Definitive Edition packs the entire Mafia II crime drama into one stunning HD package. HDR and 4K resolution are supported on PC as well as on PlayStation®4 Pro and Xbox One X consoles. Mafia II: Definitive Edition also benefits from improved facial animations to give cutscenes an even more cinematic feel. While technological and graphical improvements have given Empire Bay and its inhabitants a visual overhaul, Vito's timeless story is unchanged.
I ended up investigating the matter further and confirmed that is is as you say -- it doesn't seem to support HDR. I've corrected the article.
All the Mafia:DE games support HDR on consoles. The developerss claimed they removed it from the PC releases because no that many people have HDR monitors.
Halo MCC is included on the list despite being marked as not supporting HDR and none of the individual games seem to support it either.
The article needed to be purged to reflect the latest changes made to the Halo article. I've now purged the cache and the updated list does not include the game.