why illustrator keep auto save 1.7 pdf format when i choose 1.5 version

Illustrator 30.8.1 changes PDF 1.5 to PDF 1.7 when using multiple linked TIFFs

Hi,

I’m having a strange PDF export issue with Illustrator 30.8.1 on Mac.

My normal workflow is:

Photoshop → Grayscale TIFF → Link in Illustrator → Save as PDF 1.5

This has worked fine for a long time, but recently Illustrator started exporting PDF 1.7, even when I select PDF 1.3, 1.4 or 1.5.

I found that:

• No linked images → PDF 1.5 works.

• One linked TIFF → PDF 1.5 works.

• Adding more linked TIFFs eventually causes the PDF to become 1.7.

• I have around 70 linked TIFFs in each file, and several files show the same problem.

• Older AI files using the same workflow can still export as PDF 1.5.

I need PDF 1.5 for my printing workflow. Re-saving the PDF in Acrobat as 1.5 makes some images blurry, so that is not a good solution.

Is this a known issue with Illustrator 30.8.1, and is there a way to force Illustrator to export as PDF 1.5 while keeping the linked images unchanged?

because need to send in system I must use 1.5, anyone can help?