i had a similar problem with my XL becker tort colony and it affected all the blue torts in my system. Cali, becker, oregon torts all STN from the bottom. The recession was slow in my case and took its toll over 2-3 months. moving the corals to higher flow areas slowed the recession but it kept going until i had to frag it up and move it to my frag tank. some frags made it, others just kept receeding.
The interesting part was that no other types of sps were effected at all. its almost like a bacterial infection that only targetted tortosa? I never figured it out but I miss them. One colony of oregon tort survived. and has fully recovered.
Good luck with your battle, i would try more flow for sure since it is a large colony.