Do you see any error messages on your console in either Sandbox or AppStore environment? I assume it works in XCode environment. If this is the case, check if you accepted Paid App agreement for your sanity.

In my case, I had to sign the Paid App contract that got invalidated after my developer program has migrated from individual account to LLC recently. I discovered it while I was waiting for a response from support staff, so I updated him that the issue was resolved. In my case the storefront failed even in the Sandbox environment for both groupID and productIDs, so it may be different from you and SPEEDCUP. I'm still using groupID to load storefront successfully. See below post reply for detail.

Hey Thomssie, thanks for the head-up. I've just received a response from Developer Support Program and provided extra details to them that they asked. I will see what they say - Have the work-around treated you well so far? Did you hard-code the product ids from the AppStoreConnect - Subscriptions?

Hi, it seems that OP still has the issue unresolved, and I'm not getting any feedback from my submitted feedback either (FB14509371). Could you confirm that someone is looking at the issue?