My unconfirmed theory is that because it is tied to your warband there is a problem choosing a different pact than last week because perhaps your other warband members are still in the previous pact so the system disallows the change.
Try logging in on your other warband toons to see if upon login, the previous pact is finished, then allowing for new pact. “Unconfirmed fix theory” 
I had the same issue. I couldn’t choose a pact unless I chose the one I had last week. I assume the problem is that the quest is Warbound - I looked through my alts and found that one of them still had the quest from last week. I abandoned it in the hope that I could re-pick my pact, but I couldn’t. I assume I was stuck with the Weaver because one character was already flagged as doing the Weaver’s weekly quest.
While Blizzard needs to fix this, I recommend that anyone doing this quest on alts makes sure to finish the quest on those alts - otherwise you’ll be stuck with the same pact next week.
It’s broken for me, yes.
It just got fixed for me now. Wasn’t working earlier. Was able to select a different Pact now.
Just tried it and it worked for me, too!