I was a joint tenant with full right of survivorship on a property in Iowa. The other joint tenant filed a quit claim deed to herself in an effort to sever the joint tenancy. She died shortly after. I filed an affidavit of surving joint tenant and her name was removed from the title. Her estate opened and they claim an interest in the property. Was the joint tenancy severed when the deceased joint tenant filed a quit claim deed to herself before she died?

A quite claim to oneself when there is a joint tenant? How did she even manage to do that? You got your name on the deed again, as it should be, and so you just notify her estate of the facts, especially the survivorship. Hire a lawyer if necessary but it doesn't sound like it should be.

ANY deed needs to be signed by the conveyor of the interest. The joint tenant could deed their share to anyone they want. But the joint tenancy remains unless both tenants convey the deed. Except if the joint tenant conveyed to the other joint tenant.

But a document that conveys to yourself is moot. If they tried to convey the entire property from joint to themselves, it would be invalid without both signatures.

... Her estate opened and they claim an interest in the property. Was the joint tenancy severed when the deceased joint tenant filed a quit claim deed to herself before she died?

What do you mean by "severed"? Magically turn a joint interest into an interest in common? No, it doesn't work that way.

I don't think the QCD did anything, but look at it this way: she had no more than a possibility of acquiring the entire property if she survived you. Since she died, and you are still alive, her interest is extinguished. The QCD can not have had the effect of creating a larger interest, something by which her estate would have any interest.

What is the personal representative (the estate) doing or saying so as to "claim" some sort of an interest? What exactly are they claiming?