I can't find photos or videos that I just added to the Photos app
This is likely just an issue with the ordering of media in LumaFusion's 'All Photos' album. We are unable to sort in the same order as the 'All Photos' album in the Photos app (which is by date added to the library). We can only sort by creation date. You can find it by scrolling down through older media to find the items, you've added, but it may be easier to look in the Recently Added album.
You may also find it useful to create your own albums in the Photos app and add media that you want to use for specific projects to those albums.
You can also simply 'Favorite' any photos or videos (tap on a photo or video then tap on the Heart in Photos), and you'll find those in the Favorites album within LumaFusion (thanks to lumaluke for this suggestion)