This is the risk everyone has to be aware of. This can happen, and it can not. This is obviously a hit or miss, and crap shoot. I have encountered this too, numerous times. The next batch of strings (exact same brand, alloy, gauge) will turn out just fine, and the next one will be dead as a doornail. However, ultrasonic or soaking isopropyl alcohol (isopropanol for short) will not expand or contract the string as there's no heating involved. No cooking. I have no tuning issues or intonation issues after ultrasonic PLUS isopropanol, and the zzzing is actuall better than boiling, it lasts a tad longer.
Ultrasonic uses tiny nanobubbles that explodes (yes!) when they hit the surface, and thus everything that doesn't belong to the surface is released. You can reach corners and cracks and spaces that you can't with regular boiling or soaking in iso only. You must get into all cavitites, spaces in the strings wrap as well as the hex core spaces. This technique (ultrasonic) is called cavitation and is the on used on boat propellers and that's why some gets destroyed by the cavitation when swirling bubbles and water around. However, the ultrasonic is non-inertial which means it ain't as dangerous to surfaces.
Ultrasonic cleaning - Wikipedia, the free encyclopedia