Arbalest is going to get an inherent anti-barrier perk later this season. As is the case with other exotic weapons with inherent anti-champion mods additional anti-champion mods won't work on them. So even though anti-barrier isn't active on it yet, the blocking of additional anti-champion mods is active.

Hi there. This is because the weapon is being prepped to get Anti-Barrier as an intrinsic perk in the future. [quote]The Arbalest Exotic Linear Fusion Rifle will not work with anti-Champion mods in Season of the Lost. In a future update, anti-barrier will be added as an intrinsic perk for the weapon.[/quote] https://www.bungie.net/en/Forums/Post/259485383?page=0&sort=0&showBanned=0&path=0