In the book Elantris, the Reod was caused by a giant earthquake opening a chasm which changed the shape of all the Aons. The Seons, which are basically living Aons, were presumably unchanged, since nobody noticed the change, which should have been obvious on the simpler Aons.
Firstly, they did change:
Master_Moridin: Why aren't seons affected by the lack of a chasm line in their Aon?
Brandon Sanderson: If they tried to actualize their Aon, it would have an undesirable result. In addition, the chasm broke their bond to the humans they were tied to, and you can see the result of that. So they were affected.
We saw that Seons associated with Elantrians were exhibited some unusual behaviors, and indeed might have been considered to have lost their sentience. In fact, I'm fairly sure that their Aons did look different under close inspection, if the Seon was bonded to someone who'd be taken by the Shaod.
As to why they did, it wasn't because of the direct effect of the Reod, but because of their Connection to people who were affected by it:
Questioner (paraphrased): Why do Seons become broken when their person is taken by the Shaod?
Brandon Sanderson (paraphrased): A Seon has a Spiritual Connection with their user. When the Shaod takes the user, it messes up the spiritual nature of the user, and it really messes up the nature of the Seon.
Although the characters in the book aren't sure, it seems likely that the Seons predate Elantris, being Splinters of Devotion. As such, they're not going to be dependent on Elantris for their very existence, and the worst effects will come from their Connection to those affected by the Reod.