noise cancelling works best for constant sounds (like the thrum of an engine or a murmur of the crowd) so the anc processor can "compensate" for that sound. If a toddler was crying irregularly, with "wahs" at longer bursts, it's possible that the ANC didn't have time for it's time window to compensate for the sound. Aka the sound started and stopped before a particular time window.

not sure if it is by design, but a larger window would potentially decrease computational load since it doesn't need to recalibrate and reprocess at a higher rate.