I remember how a lot of folks wanted so badly for Flayn to be in FW just because Seteth was there. Even though all the evidence pointed to it being a prequel (remember that after the War of Heroes, Flayn fell into a deep coma that she only awoke from shortly before the events of Three Houses), everyone kept leaning towards sequel for some reason.

Paperluigi30 said in comment #2634426:

everyone kept leaning towards sequel for some reason.

Probably because they assumed that older looking Sothis= actually older Sothis.
As if that was ever a reasonable assumption in Japanese media.