The max time default for ReplSetMonitor::getHostOrRefresh is clearly too low at 15 seconds, given failover in our testing can easily take that long on a slow machine or just timing election runs just right.

 
const Seconds ReplicaSetMonitor::kDefaultFindHostTimeout(15);

However, ReplSetMonitor::getHostOrRefresh timed out after 20 seconds in the linked BF, so this suggests a caller is passing in 20 seconds, which again is too short to be reasonable for testing.