To reply to my own post: the problem I was experiencing was resolved in much the same way as for josessolmaximo: I explained that I thought there might be a problem on AppReview's side since the IAP process worked perfectly against the sandbox, and I quoted this thread. App Review then asked me to provide a video demonstrating the full IAP process running against the sandbox on the latest version of iOS/iPadOS. After receiving the video the app was taken back into review and subsequently approved.

Interesting. This is currently happening to me: it works perfectly in development against the sandbox and when using TestFlight but consistently fails in review. I raised a TSI for it and was told the same as you, that SKProductsRequest was returning no products. A bug was raised for the App Store Server engineers to investigate, but the problem is still present a week later. Unfortunately App Review appear to be ignoring anything I put in the review notes or send to them via the submission messaging system. I'm literally at the point of abandoning the IAP and going for a paid app because there doesn't seem to be any other way out.

Same thing here as well. It shows up when I build a project that isn't set up to use Xcode Cloud.