Hey there,
My guess is some integration-related issue between the Facebook & Instagram sales channel app [I am assuming you have it, given other parts seem to function normally] and your Shopify account. I am also assuming that the events are being fired correctly, too, because you got the Tag Manager and that should be recording accurate data.
While this may be a little tricky, it seems like there is a setup issue. This might need more scrutinizing and checking where the problem is; I can recommend a quicker way to display your analytical data on Shopify by using a Shopify app. One that tracks your events and provides an in-depth report is AdNabu’s Meta & Facebook Pixel.
I mean, you can easily evade the issue by using a more streamlined way of tracking measurements. The analytics appear in the app while you’re on your Shopify admin. That way, you can see the marketing measurements on Shopify.
I hope this helps in some way
!