This has been the same for me as well. We’re all getting screwed unless we get all the reputation we missed in the next week or so. That and double the XP we get for catch up with this bug.
It’s fixed. I just finished it right now.
If you started the campaign already and are not close to hitting a rep requirement for the next quest your best bet is to start on a new timerunner who hasn’t started it yet.
Verified, I’m still short on my Warlock but I restarted it on my Evoker and in 10 minutes Im further along than my Warlock is after 5 days.
Hmmm I will have to try it again. but starting a new time running is by far the biggest slap in the face if I had to do that.
I’ll report back on my main timerunner (this one) to see if it’s fixed.
Edit: only getting 300 rep for daily, 500 rep for coin turn ins. 
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This is the biggest of lame
I was able to make it work, just push current rep over into the next one (friendly to honored) by spending charms, then was able to complete the campaign and also reached exalted at the same time.
Adding my voice to this disappointing problem. I came to Remix late, got really excited about it, and can’t do this achievement. I grinded every class to 70 specifically for it, too. At this point I hope they’ll simply remove the achievement requirement and let people buy the transmogs. I would have happily put in the hours if the achievement wasn’t bugged.
I believe it was fixed a couple weeks ago, if you didn’t get it done I am sorry. Bugs like this really need to take top priority for limited time events.