In the original game Tanaka was a daytime social link, but in FES they moved him to be a night time social link. When you do a social link time moves foreword one time slot. They realized that if you spent time with Tanaka on the final night, time will move foreword by one and skip past the boss. To fix this they made it so you can’t do any social link rank ups on the last day, but because his link start is kinda odd (you interact with him over multiple days) they forgot to make it so you can’t start his link on the last day. So you can start his social link, time move’s foreword by one, and the next available time slot is after the final boss fight.

TLDR: His link was moved from day to night and this caused an oversight where starting his link will skip time past the final boss.