Update: I replied to the reviewer with videos of me completing the purchase in sandbox and suggested again this could be an issue on their end. I received the reply Thank you for making these changes. We will continue the review, and we will notify you if there are any further issues. My app was then approved despite the fact I've made no changes. I assume this must have been an issue with testing?

Also experiencing this. I submitted my app for initial approval with the subscriptions attached, and the submission was rejected. I made the requested changes and resubmitted, and the release was eventually approved. However, I checked my subscriptions and they had been rejected. I made some updates to them (I had no feedback on why they were rejected) and resubmitted for approval. Since then I've had another app release approved but my subscriptions are still "Waiting for review".