I guess the title rounded up to 100%, but the reward wasn’t given until the last POI was found?
For me I already had all the POI (I joined the order of shadows so got the Chantry of Secrets POI a long time ago) but I still had to go back to the Chantry at which point it awarded me an empty chest, and gave me the gifts of exploration. Until this latest patch I’d been stuck at 97% on the achievement despite having 100% completion (even had all region explores complete and all puzzle achievements just to make sure lol).