Currently Cloner::copyIndexes, on primary builds indexes w/o using indexBuildsCoordinator thread pool, but generates start and commit indexBuild oplog entry. This can lead to primary and secondaries disagreeing on indexBuildsCoordinator worker thread availability. consider a case, its a 2 node replica set and thread pool size is 1 and if we get oplog sequence some like below.

1) startIndexBuild for index x_1 on foo.bar ==> started as part of Cloner::copyIndexes,
2) startIndexBuild for index y_1 on some.random ==> started by user using createIndexes command.
3) commitIndexBuild for index x_1
4) commitIndexBuild for index y_1