I’ve spoken personally to a few designers from well known cable makers in home audio, and the consistent message I heard is a good cable is in one way like a high grade capacitor, it has a dielectric that needs to form before the sound stabilizes. When new, the dielectric absorbs and re-releases signal causing a smearing of high frequencies. Once the dielectric forms this smearing is no longer present.
With IEMs, my burn-in procedure has more to do with getting time on the cable, less with time on the drivers, especially BA drivers.