Hello,
This happened on Oct 26 and after I reported in game and to support team, they said they could do nothing about this.
So I post here to see if some other people get the same bug as I did.
On Oct 26, I was in Mad King Says event three times in total:
1) the first time
I waited on the Plaza near the chest before the event started, but just before the event started I left my computer without logging out.
So my character was on the plaza but I didn't participate in the event.
When I came back, my character was near the forge. So it looked like Mad King counted me in his event and gave me "little something" to port me into the forge. (Guess because I was on the plaza during the event. When I "didn't" do emotes, it still gave me buff.)
Other than that, I didn't get any item. No chest on the right lower corner as if completing the event successfully. I didn't know if I got karma and gold as I was not in front of my computer at all.
So I thought I didn't complete Mad King Says in this round, I came back to the Plaza later.
2) the second time
I participated in the event as usual, finished with 15 layers of buff.
I got the chest at the end and was also ported to the forge.
But there was only 2 key fragments in the chest instead of 3.
It didn't quite make sense so I reported with in game support.
And I also came back later for the next round of Mad King Says.
3) the third time
I participated again also with 15 layers of buff at the end.
This time I only got karma and gold, no chest at all.
In my inventory, since Oct 26, I got these 2 key fragments that can never be made into key because when I complete more Mad King Says later, it won't give me the missing key from Oct 26 from the first round of Mad King Says when I was not in front of my computer.
And the support team said they could do nothing about this. I can only do more events to get key fragment which will never replace that one eaten by the bug.
For some unknown reason, the system gave me only 2 key fragments into my inventory on Oct 26 while somewhere in the system record it felt it had already given me 3 and playing more Mad King Says on the same day didn't give me the third one.
So I guess the lesson is don't idol near the plaza during Mad King Says because Mad King might just eat the key fragment like this.
And if that happened, there is no way to get this missing key fragment back.