Strange disappearance of an element when saving to printable PDF format

Hey, guys! 🙂 The following thing happened to me. I created a 24 page A5 brochure in Adobe Illustrator, with graphics and text running across the middle of the page. After I saved the PDF version together with the bleed, one important element that was close to the center was deleted in a mysterious way. It just wasn't in the PDF.

Of course, nobody (neither me nor the other person who sent it to the printers) noticed anything and misprints were printed. 

Then when I went back to Adobe Illustrator Original, the element was there, so I tried saving it again and it was still there.

Is there any logical explanation as to how something like that could happen? Fortunately it was all taken lightly by the customer, but in the future I would like to know how to avoid such a situation.

Does anyone know why something like this could happen? I am very grateful in advance for your answer and sharing your experience! 🙂