Image stored in documents directory disappears on app relaunch/rebuild

I am relatively new to swift and completely new to FileManager, and I encountered some very strange behaviour of my app. I have a "Day" struct that can save an image provided to it by the user:

struct Day: Codable, Comparable, Equatable, Identifiable {
      /*
     some vars and inits
     */
     
     private var dayImagePath: URL?
     
     var displayedDayImage: UIImage? {
        guard let dayImagePath = dayImagePath else { return nil }
            
        do {
            let data = try Data(contentsOf: dayImagePath)
            guard let image = UIImage(data: data) else { throw ImageProcessingError.noDayImage }
            return image
        } catch {
            print(error.localizedDescription)
            return UIImage(systemName: "questionmark.folder")
        }
    }
    
    mutating func addDayImage(_ image: UIImage) {
        
        if dayImagePath == nil {
            dayImagePath = FileManager.documentsDirectory.appendingPathComponent(id.toStringId().appending("DayImage.jpeg"))
        }

        if let data = image.jpegData(compressionQuality: 0.8) {
            do {
                try data.write(to: dayImagePath!, options: [.atomic, .completeFileProtection])
            } catch {
                print(error.localizedDescription)
            }
        }
    }
    
    /*
     some more code here
     */
}

The documents directory is found using this FileManager extension:

extension FileManager {
    
    static var documentsDirectory: URL {
        let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
        return paths[0]
    }
}

Adding the image works well, the image is displayed and stored correctly. However, it gets lost on the next app relaunch/rebuild with the following error message: The file “20220917DayImage.jpeg” couldn’t be opened because there is no such file. What am I doing wrong? I would be very grateful for any suggestions. Thank you!