SERVER-114237 introduced a new sampling mechanism for the change streams pre-images collection that is (currently) based only on timestamp value.
The sampling mechanism as it is right now does not take the applyOpsIndex part into account when determining the steps for the sampling.
Thus it produces suboptimal results if there are ranges in which many documents have the same timestamp value, but different applyOpsIndex values.
The sampling algorithm should be improved to take the applyOpsIndex values into account, too.