Bravo! Yes, it's the presence of @module that's the problem. Well caught. And it worked for me on the non-TEI namespaced <correspAction>. (But note that type="semi" is perfectly reasonable ... yes, the value list occurs in the schema "only" as comments, but a processor may read those comments and, e.g., make a pop-up list as oXygen does.) (And also note you don't need mode=replace on the <valList>, I suspect because it's not that att.typed has an empty <valList> you are replacing, but rather it doesn't have one.)

I note the following things in the definition for <correspAction> in the corresp ODD

1) you should not use @mode="add" together with @module on an <elementSpec> -- it's meaningless, since you are not allowed to add new things to existing TEI modules

2) if you want your valList to appear in the schema you must put @type="closed" on it. otherwise your suggested values appear in the schema only as comments

3) since you are replacing the existing (vacuous) declaration for the possible values of @type, you must put @mode="replace" on your valList

With these changes, things work as expected for elements in the TEI namespace. I am still puzzling over why they don't for non-TEI namespaced elements, but have to go out now.