insertOplogAndUpdateConfigForRetryable() is only called for the first op='i' oplog entry in the group.

if (op.isForReshardingSessionApplication()) {
    return insertOplogAndUpdateConfigForRetryable(opCtx, entryOrGroupedInserts.getOp());
}

https://github.com/mongodb/mongo/blob/cc8671bcf846a6aeda28a5cf81aea750c2fca4bd/src/mongo/db/s/resharding/resharding_oplog_applier.cpp#L409

Note that ReshardingOplogApplicationRules::_applyInsert_inlock() throws when the repl::OplogEntryOrGroupedInserts is a grouped insert to cause repl::OplogApplierUtils::applyOplogBatchCommon() to retry with the op='i' oplog entries ungrouped.