I have a few cases of discrepancies between the conversion tracking in my Facebook campaigns reporting and the UTM parameters reported in my Shopify Admin.
An example is an order I got on April 26. In Shopify, the order has the following UTM parameters:
Campaign: Website Traffic
Medium: Social Adv
But in FB Business Manager the order is tracked as converted through one of my retargeting campaigns. I understand that there can be differences between the tracking of Google and Facebook, but I can not think of a case where the UTM Parameters are different than the conversion of the FB Pixel, as I would guess that the UTM parameters are attached from the last link my customer clicked - the retargeting ad.
This likely has to do with the methodology of how the attribution works on both platforms. At the end of the day, the UTM source will be more accurate, meaning the user definitely came from your original ad (not a retargeting campaign).
However, it is possible that retargeting campaign is the last ad that was viewed, meaning that FB has attributed that user to the remarketing efforts.
It's a wonky system, they don't always play nice. If you want to chat more about it, reach out at UriWeinber@gmail.com
Are you running regular retargeting or Dynamic Product Ads?