Google Tag Manager doesn't fire on 404 pages

We just moved from vanilla forums to Discourse and it is great - not sure why we didn’t move a long time ago.
We are using google tag manager for a variety of functions but it appears that google tag manager doesn’t get initiated on a 404 page.

Here is the live site: https://forums.paddling.com
If you view the source or inspect, you can see that the GTM container is called.

However, if you load a page that creates a 404, the GTM container is never loaded.
https://forums.paddling.com/foo_bar/