Ich hatte ja das selbe Problem und ich hatte das auch dem Shopify Support geschrieben. Die haben das für mich gefixt, hier die Antwort vom Support:
!Firstly, I would like to apologise for the misunderstanding that has clearly happened in this thread which has inevitably prolonged the issue for you.
I understand your requirements to display the unit price for your products, everywhere that the price is displayed and this of course should also include your checkout page.
Upon examining your store settings on our side, I was able to determine that the Beta tag for this specific unit price feature was in fact not enabled on your store.
I have subsequently enabled this feature for you and tested your checkout. I am pleased to report that the unit price is now active and visible on your checkout page"
Danach hat es tatsächlich funktioniert, die Erklärung vom Support war für mich aber nicht ganz verständlich. Vielleicht hilft dir das weiter?