From https://jira.mongodb.org/browse/BF-41017

This part of the test set a failpoint to wait for 200ms(by default) before each yield. Run a find command touches 4 documents with batchSize 3.

The test waits until it hit failpoint 3 times then run shardDb.currentOp on the shard. Wait for operation to finish. Then it first checks the cumulative counter on router and the find ran on router is >= 1. And it fails when checking curOp to see if there was 1 find running.

    failPoint.wait({timesEntered: 3});
    const curOp = shardDb.currentOp({"command.comment": findComment, "command.find": "testColl", "active": true});
    joinShell();

    // Ensure that serverStatus indicates a find() was run.Add commentMore actions
    {
        const serverStatus = routerDb.serverStatus();
        const findMetrics = serverStatus.metrics.commands["find"];
        assert.gte(findMetrics.total, 1);
    }

    {
        assert(
            curOp.inprog.length === 1,
            "Expected to find exactly one active find() command with the comment " + findComment,
        ); <----- the assert failure

This looks very much like a timing issue. After find op hit failpoint 3 times, and before shardDb.currentOp read the status, there's possibility the find op completed on shard, therefore there's no active find command.