up to https://docs.openshift.com/container-platform/4.13/support/gathering-cluster-data.html#gathering-data-specific-features_gathering-cluster-data the image "registry.redhat.io/rhmtc/openshift-migration-must-gather-rhel8" should get used to gather data for support cases.
As this image is not a related image in the operators spec, this image will not get automatically pulled/mirrored. 
I case of a disconnected cluster this leads the related must-gather image likely not being available. As this get mostly not discovered during usual usage, but in case of a crisis, this needs to get avoided 

all, AFAIK

always on any disconnected cluster or by checking the redhat-operator-index

1. opm render registry.redhat.io/redhat/redhat-operator-index:v4.13 > redhat-operator-index-v4.13.json
2. operatorNames=$(jq 'select(.schema == "olm.bundle") | select ( .relatedImages[].image | contains ("registry.redhat.io/rhmtc/openshift-migration-must-gather-rhel8")) | .name' redhat-operator-index-v4.13.json ) 

no operator has "registry.redhat.io/rhmtc/openshift-migration-must-gather-rhel8" as related image

"mtc-operator*" is expected to have a related image "registry.redhat.io/rhmtc/openshift-migration-must-gather-rhel8*"