Actually, Nynaeve heals the wounds that are left. It is explicitly mentioned that when she pulls the thorn from Naeff's mind, she heals the hole left behind.

Other than that however you are likely correct and there is something left that would allow the taint to consume the victim quickly after healing. Nothing else really makes sense. Other than the idea that the fabled AoL knowledge/skill of OP was just a huge hoax and they were actually all complete idiots.

Infact I have long been annoyed that healing someone who has been stilled/gentled was supposedly something that was unknown in the AoL, but so easily discovered by two separate totally inexperienced healers who did not even work together. The AoL'ers should have EASILY been able to discover that and heal someone who had been severed from the source.