Google Ads Conversion Tracking only firing on some Leads from AMP HTML

I've successfully set up Google Analytics & Google Ads Conversion tracking but we're having some inaccurate data being passed into Google Ads.

We have a Conversion Goal in Google Analytics for the Thank you page. This is being tracked perfectly on every source users come from to submission. This suggests there is nothing wrong with the form.

Google Ads is set up with manual tracking through UTM parameters. This is where the issues occur as we have two conversion goals on this.

1. Thank You Page Conversion
2. AMP Form Submit (Using the 'amp-form-submit-success' as the event so it only fires once)

These two conversion types are very inaccurate and always have fewer conversions than the realistic number within the CRM.

Is there any reason why only a few of the conversions are being passed into Google Ads? One thing I have noticed is that the UTM parameters are being removed when users land on the landing page. I can see this as the data is not being passed into the CRM on a lot of leads.

Out of 16 Leads From Google Ads/PPC:

• 3 appeared with UTM_Term, UTM_Campaign, UTM_Source and UTM_Medium through to CRM (The rest blank parameters).
• 5 Showed a Thank you Page Conversion Goal.
• 3 Showed a Form Submitted Conversion Goal.

Google Form Submission Conversion Tracking below as a reference, although this is working fine as some leads are coming through. I've checked all the AMP Cache API URLs and they are caching the correct pages.

Is this something to do with the Google AMP cache. Any guidance to documentation or solution to get more accurate conversion counts into Google Ads will be much appreciated.

 <script async custom-element="amp-analytics" src="https://cdn.ampproject.org/v0/amp-analytics-0.1.js"></script>
    <script async custom-element="amp-form" src="https://cdn.ampproject.org/v0/amp-form-0.1.js"></script>
    <script async custom-template="amp-mustache" src="https://cdn.ampproject.org/v0/amp-mustache-0.2.js"></script>

    <!-- Global site tag (gtag) - Google Ads: XXXXXXXX -->
        <amp-analytics type="gtag" data-credentials="include">
        <script type="application/json">
        {
          "vars": {
            "gtag_id": "AW-XXXXXXXX",
            "config": {
              "AW-XXXXXXXX": {
                "groups": "default"
              }
            }
          },
          "triggers": {
            "C_LZsYO74kXME": {
              "on": "amp-form-submit-success",
              "request": "event",
              "selector": "#bdf-amp-form",
              "vars": {
                "event_name": "conversion",
                "send_to": ["AW-XXXXXXXX/XXXXXXXX"]
              }
            }
          }
        }
        </script>
        </amp-analytics>

It was as simple as the non-amp version script not firing, nothing to do with AMP