How are you sequencing the loading of the Forms 2.0 library (forms2.min.js) and the GTM-injected script you’re using for UTM tracking?
Looks to me like you have an obvious race condition.
That UTM tracking script only checks for MktoForms2 once, when it runs, instead of waiting properly for MktoForms2 to be available. As a result, it runs as if there’s no form on the page.