Code injection not injecting on every page

Hi,

So I'm trying to connect Google tag manager to Squarespace through the code injection tab. And it works on some pages, but on others (crucially the checkout page) it goes missing. The issue is definitely not with the GTM snippet, because I tested with a simple console.log statement and that doesn't show up either.

The way I added the code is through Settings > Developer Tools > Code Injection, and pasted it into the header section.

Does anyone know what I'm doing wrong? The checkout url is checkout?cartToken=XXXXXX if that helps.

Thank you