Marionette kill didn't count
We’ve had the exact same thing happen to us. We dc’d early on in the event, came back, did our lane. When the marionette got killed, we got nothing.
It seems like, if you dc at any point during the event, even if you sever a chain afterwards, the whole thing stops counting.
I’ve done this fight twice today, and neither tries triggered towards the achievement. I didn’t experience any dc’s or lag during the encounter.
The last fight I did (at 12pm pst) we were at 3/5 chains severed, I didn’t get a chest either.
Just had the same problem with Marionette – first time successfully killing her and i got nothing, no daily completion, no chest. Had to quickly relog (only to character select) at one point to unfreeze my skills but got back into my lane and finished the fight.