We've had several failures from the :qa:mixed-cluster:v7.8.0#mixedClusterTest in a short period. Two examples are:

https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob+fast+bwc/4543/console
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob+fast+bwc/4542/console