In https://jira.mongodb.org/browse/SERVER-47828 we switched from returning a member configuration by copy to returning a pointer. Unfortunately that pointer is to data protected by the repl mutex and can change. We can't switch back to a copy (too slow, which was the original reason for the ticket) so we'll need to come up with another solution.

It's possible there's a bigger issue, in that I'm not sure how the index build coordinator handles it when configurations change while it's running. We may need something like an external config lock.