it looks like the master/4.6 is still on https://github.com/openshift/cluster-csi-snapshot-controller-operator/blob/master/go.mod 1.18.3, please update at least to 1.18.6
Solved by rebase to external-snapshotter v3.0.0 that brings k8s.io/client-go v0.19.0
According to comments "It looks like the issue only affects 1.18. According to https://github.com/kubernetes/kubernetes/issues/91073#issuecomment-652251669 the issue was first introduced in that version by changes done to the reflector. The fix is already present in 1.19."
In the cluster-csi-snapshot-controller-operator repo:
$ git checkout -b 4.6.0-0.nightly-2020-09-24-211545 c536c8d4165e81f6c9c81de0627aeed6fc57bb5c
$ grep 'k8s.io/client-go' go.mod
Change status to VERIFIED.
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (OpenShift Container Platform 4.6 GA Images), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.