When I try to add shape keys, it change the vertex order

I’ve been trying to create a custom morph for a Daz figure, but I’m having a problem with Blender messing up my vertex order when I try to use shape keys.

If I bring two OBJs into a scene (with “keep vertex order” selected), then go to shape keys and do a “join as shape key”, then export as an OBJ (with “keep vertex order” selected), the vertex order of the new OBJ no longer matches the original, in fact Blender tells me that even the number of vertexes has changed.

I went through a few tests of importing-exporting at different stages to narrow down that this is definitely happening right the moment i join the meshes as shape keys. Does anybody know why this is happening and how I stop it?