Lost photos

What exactly do you mean with 'changed'? Do they have a different file extension? That would make Lightroom lose them. Do they still have the same file extension, but are they shown in the Finder as belonging to Affinity? That should not matter to Lightroom, it should still be able the handle these photos just fine. Lightroom will also lose the connection to a photo if Affinity moves or renames it.

I don't know anything about Affinity. Photos are not "in" the Lightroom catalog. The catalog simply keeps track of where the images are located on your computer hard drive, as well as all the adjustments you have made to those images. If Affinity has moved those images or has changed their attributes so that Lightroom no longer can recognize them as they were imported then there isn't really much you can do. LrC is looking for the name of the image as it was named when it was imported, and is looking for that image in the folder where it was imported.