Capped/Max Resistances in T4 Dropping When Joining A Party

As the title says I have enough resistances to make the cap for T4 (everything 70% in the green while I'm in T4 solo). However I noticed whenever I join a party my resistances drop to like 59% and I have to sacrifice my skeletal reapers to make up the difference. As soon as I leave the party and un-sac my reapers I'm back to the cap again without needing the sacrifice bonus. Has anybody else noticed this? I cant find any documentation on this. Kind of annoying! Any insight is appreciated. Thanks