Actually I had similiar numbers before. Judging from my list of shards, it occured during x2.
The count starts with the last void drawn being an epic. So the epic mercy overall is 0.
It occured 8 times for me, that I pulled 20+ void shards in a x2 event without getting an epic.
The worst was 14 shards followed by 21 shards followed by 28 shards before getting an epic. You could say that I got 4 void epics from 63 shards. If I just obmit the first and last void, I would have been standing there with 2 void epic from 61 shards, if I am concentrating on the worst scenario I would get to 1 void epic from 48 shards. It is possible and very frustrating.
The highest number on void shards on x2 without epic on my list is 29
Btw...the numbers according to my charts for ancients (x2) is 12 times 20+ shards without epic and highest number of shards without epic being 28.
At the same time, I never had a double pull of leggos on a 10-shard pull...neither ancient or void.
So it is very possible to occur....