After Seiji and the others became members of the Shadow Guard, the Third Mizukage did not revoke their Anbu designation.
Strictly speaking, the Shadow Guard could also be considered part of the Anbu. They simply no longer carried out Anbu missions.
The Shadow Guard was essentially Anbu directly under the Mizukage, tasked with personally protecting the Mizukage and carrying out the Mizukage's orders. They answered to the Mizukage alone.
Furthermore, Shadow Guard members were permitted not to wear Anbu masks or uniforms. Back then, Seiji's father, Kaguya Genji, had simply worn the standard attire of the Kaguya Clan and never worn a mask, since he was too well-known and the combat style of his Shikotsumyaku was far too recognizable.
Thus, among the Seven Ninja Swordsmen, only Seiji and Kushimaru Kuriarare still wore Anbu masks. Seiji could not yet reveal himself publicly to everyone, while Kushimaru Kuriarare had simply grown used to wearing his mask and did not want to take it off.
Time passed quickly. More than half a year had passed since the Seven Ninja Swordsmen of the Mist had been established. During that time, Third Mizukage Hyou had spared no expense in training resources and ninjutsu, determined to forge the Seven Ninja Swordsmen of the Mist into a force surpassing Iwagakure's elite demolition corps and Kumogakure's once-renowned Kinkaku and Ginkaku Unit.
Over those six-plus months, the other six had all become proficient in using their respective ninja blades. Empowered by the blades, every one of them now possessed combat strength steadily above that of a Jonin. Seiji, meanwhile, had relied on his extraordinary talent to become proficient with and wield all of the ninja blades.
In October of Mist Year 35, one year had passed since the outbreak of the Second Shinobi World War.
Sunagakure and Konoha