Amazon link changing its code after I’ve saved it??

Hi

I have a site https://www.digital-luggage-scales.co.uk that I intend to develop for affiliate work.

The problem I’m having is that the page validates for the Amazon code (when I put amp; after the &) BUT it only validates for so long. For some reason after a period of time (and it varies betwenn minutes and hours) the code reverts back to using no amp; before the & and therefore doesn’t validate. The theme designers say it isn’t a problem with their software.

Any ideas before the keyboard gets it??