The fix to add custom CAs was already backported to the 3.9 installer in this PR: https://github.com/openshift/openshift-ansible/pull/9338 The manual workaround for this issue would be to append the custom CA to /etc/origin/master/ca-bundle.crt on the master node.
Closing this because the fix was already merged into the 3.9 branch as part of the previous ticket.