I am also on v21.03. That is interesting though, I guess I wasn’t quite as lucky with my upgrade. 
Edited:
Hmm, looks like the same issue popped back up. I can do the same “trick” again and it seems to temporarily fix the issue.
If helpful:
What version of Dgraph are you using?
Dgraph codename : rocket
Dgraph SHA-256 : b4e4c77011e2938e9da197395dbce91d0c6ebb83d383b190f5b70201836a773f
Commit SHA-1 : a77bbe8ae
Commit timestamp : 2021-04-07 21:36:38 +0530
Branch : HEAD
Go version : go1.16.2
jemalloc enabled : true
For Dgraph official documentation, visit https://dgraph.io/docs.
For discussions about Dgraph , visit http://discuss.hypermode.com.
For fully-managed Dgraph Cloud , visit https://dgraph.io/cloud.
Licensed variously under the Apache Public License 2.0 and Dgraph Community License.
Copyright 2015-2021 Dgraph Labs, Inc.
Have you tried reproducing the issue with the latest release?
Yes
What is the hardware spec (RAM, OS)?
NAME="Ubuntu"
VERSION="18.04.5 LTS (Bionic Beaver)"
For development, I am just running one zero and one alpha on the same computer with 32 cores, 64GB ram, and 500 GB nvme.
Steps to reproduce the issue (command/config used to run Dgraph).
Still figuring this out… Seems to be related to certain indexes getting heave read/writes