So this is an ongoing issue.
Here's the backstory: By default, every app on iOS/iPadOS has a file storage area that belongs to just that app. When you transfer files to an app using any of the usual iOS tools, they go into that space, and then that app has access to the files. The problem comes when the developer adds an AUv3 plugin to their app. Because of the way Apple designed the OS, the AUv3 plugin doesn't have access to the same file bucket that the standalone app does. This is, of course, a huge problem. The workaround is to create a "group" bin that both the standalone app and the AUv3 have access to. This is what I have done.
But the problems don't end there, if you try to send files to Decent Sampler, they go into its regular file directory...which Decent Sampler doesn't use for file storage. So...yeah. Not a great situation. The solution is probably for me to have the app move any files it finds in the regular file storage to the group file storage area every time. I will do some research to see what my options are.