In versions 4.4 and up, we support two-phase index builds meaning that a primary will wait to fully commit an index until the index is built on all secondaries. This is not the case on 4.2; secondaries will not build the index and effectively ignore the "startIndexBuild" oplog entry. 

There is a race condition on the primary when setting the index build protocol (single/two-phase) and downgrading the FCV which takes a global lock (whereas setting the protocol does not). So, if the index build protocol is set to two-phase and then we downgrade (on the primary), and then the secondary downgrades before building the index, the primary will wait for a response from the secondary signifying the index was built, but the secondary (on version 4.2 with the single index build protocol) will not build it.