the iXpand app does not change the file name. The default name iOS uses would be IMG_XXXX.jpg.
What photo editing app or camera app are you using? It seems whatever you are using to edit or take the photos is saving the multiple files with the same file name.
iOS will allow multiple files with the same name. The file system on the iXpand (exFAT or FAT32) does not support multiple files with the same file name. If this is the case you would need to rename the files to with unique names before you transfer them.
To test this take a few test pictures using the default camera app on the iOS device. Then use the iXpand drive app to transfer those photos. They all should transfer with the name formatted IMG_XXXX.jpg where XXXX will be the sequential number the photo was taken.