It’s a result of the somewhat peculiar way iOS handles “open with”. When you download the file from your Google drive it’s written to temporary storage on the iPad. When you open it in Audulus the temporary file is passed to Audulus. While Audulus is open the file will be visible in Recents but when you close Audulus the temporary file will be deleted. You need to do a long press on the file icon in recents and move it to the Audulus folder.