I don’t think it matters what theme people use because they are hosted
checkout thank you pages. Correct me if I’m wrong.
From my investigating, the problem lies with Shopify issuing a redirect on
the thank you page. And then not triggering the conversion tags on the
order status page where the user is forwarded to.
To get round this I’ve made a custom liquid if statement which fires a
conversion tag but only If the order time and date is within five minutes
of the current time.
That stops any duplicate conversions being tracked.