Oldest timestamp not always advanced with --enableMajorityReadConcern=false, on secondary nodes

Run the following against a replica set with majority read concern disabled. It generates bursts of updates spaced 15 seconds apart.

function update() {

    for (var i = 0; i < 1000; i++)

        db.c.insert({x: 0})

    for (var x = 0; ; x++) {

        db.c.updateMany({}, {$set: {x: x}})

        if (x%20 == 1)

            sleep(15000)

    }

}

FTDC shows the following:

During the bursts we are executing a couple hundred batches per second, but the oldest timestamp is not advanced on every batch, and in fact may not be advanced for minutes at a time. This can cause a large amount of data to be pinned in cache.