When I run JHove on a valid JPEG, created by an iPhone dual camera, an error is thrown and the file is marked as not well-formed.

This is the JHove output:

➜ jhove -m JPEG-HUL IMG_1616.jpg 
Jhove (Rel. 1.32.0, 2024-09-12)
 Date: 2025-07-04 00:13:27 CEST
 RepresentationInformation: IMG_1616.jpg
  ReportingModule: JPEG-hul, Rel. 1.5.4 (2023-03-16)
  LastModified: 2025-07-04 00:12:54 CEST
  Size: 3723240
  Format: JPEG
  Version: 1.01
  Status: Not well-formed
  SignatureMatches:
   JPEG-hul
  ErrorMessage: Expected marker byte 255 : Read marker byte value 65 
   ID: JPEG-HUL-7
   Offset: 21
  MIMEtype: image/jpeg

On this sample image: https://www.jianguoyun.com/p/DZou8ewQgsqOBxjb-Ho

Below is a screenshot of the hex-editor of the first few bytes:

APP0 starts at position 0x02 with marker '0xFF 0xE0' => OK
next word is at 0x04 -> 0x14 = 20 => OK
So next section is APP1 at postition 0x04 + 0x14 = 0x18, '0xFF 0xE1' => OK

But JHove starts reading the section at offset 21 (0x14) => Not OK

I found the code at

Apparently, in this case Apple has added the 4 bytes 'A', 'M', 'P', 'F' to the end of the regular APP0 and added 4 to the length value of the APP0. This makes this JPEG a bit special, but still well-formed.

See also the remark in the ExifTools code at https://github.com/exiftool/exiftool/blob/a755221b7937406a16b40bcd533e992aa8b5caa0/lib/Image/ExifTool.pm#L2251