75% Damage Reduction Tank Datacron Level 6 has 2 Bugs

Jabba's lead has more damage reduction for hutt cartel which adds together with the datacron, causing the 1 damage you've seen. 

Don't know about the subsequent hits damage reduction though. Edit: just thought, does the assist call have a damage reduction built in? Like "all allies assist doing 30% less damage". That could add together with the datacron, just like jabba's lead.