So sorry you are having a problem exporting. It's not a normal thing.
However, I would recommend turning OFF "Optimize iPad Storage" if at all possible. The difficulty is that Apple is managing what is stored on your iPad versus what is in the cloud (in which case only proxies are shown on the device), and they don't give us any way of locking the used media onto the device to keep it local. If the app is in the foreground and your project is open, then we have a list of what is being used and the device SHOULD hold onto the media.
Unfortunately, we they don't give us any way of knowing if the iCloud media is there until we try to access it. We do checks for media presence (by accessing the used files) when you start the app and before export, but I'm wondering if the reason you are getting failures could be that at some moment the media is inappropriately re-optimized in the background and we loose it.
If you were to import media from Dropbox for instance, then the media is stored with our app, and it cannot be optimized away. We have considered always copying media you use form iCloud into our app, but this will result in a huge app that you have to manage.
In any case, since we have joined you to the beta we can get your project and restore it and it might even be worth you restoring it on your own device, since in this case LumaFusion will hold the media and it will not be able to be optimized away by Apple.