UTM Tracking on Email Smart Campaigns Marketo

Hi,

I'm wondering if anyone can help with an issue I'm having with UTM tracking on my emails being sent from Marketo in smart campaigns. 

I am syncing emails from Knak into Marketo.  I can see in the email template that the UTMs are pulling through from Knak correctly, and in the preview when I hover over images etc I can see the UTMs populating correctly. 

However, when I send a test email (either from the template or from a live test campaign), the UTMs on some links aren't pulling through, but oddly are working for others. 

I noticed that the UTMs are visible in the URL of links going to one particular webpage, but for all others the UTMs are being replaced with 'email.perenews....'.  I believe this is Marketo's own function overriding my UTM tracking, however it doesn't make sense why that's only happening on some links and not all. 

Is anyone able to assist in understanding why this is happening, and how to fix it?