I found that when an ECM dies, all additional missiles which were attracted to it return to their original target rooms, but they do not do any damage if they hit. I've confirmed this for both volley wpns like HAL and RAL, as well as penetrator missiles. and I've confirmed neither hull, sys, crew, nor AP damage types are being done by these missiles. a "hit" animation plays, but there is no text for damage done.
Note, I've confirmed that tracking is applied correctly on both hits and misses for these "additional missiles" which were attracted to ECM, then redirect to intended target.