SES won't tag links with unique identifiers

Hello, I'm trying to uniquely add identifiers to links, so that I can track Click events separately.

I'm following what described in question 5 ("Q5. Can I tag links with unique identifiers?") of the Amazon SES email sending metrics FAQs, which suggests to add ses:tags:

<a ses:tags="product:book;genre:fiction;subgenre:scifi;type:newrelease;" 
    href="http://www.amazon.com/…/">New Releases in Science Fiction</a>

The problem is that, upon reception of the email, the ses:tags do not appear in the email received by recipients. Even by inspecting the source of the email delivered, it is clear that these tags have been stripped before delivery (still not clear to me if the stripping has been done by SES or maybe Gmail).

(To give additional context: generic Open and Click tracking work fine with my current setup. Only ses:tags are not showing up).

Can anyone help please?