Here's an update to our situation. We never figured out what the cause of the issue was. We just submitted a new binary for review and this time it passed the review without any issues.

We found that your in-app purchase products exhibited one or more bugs when reviewed on iPad running iOS 14.4 on Wi-Fi. The app hangs on an indefinite activity indicator after tapping “Subscribe now”. Our app was rejected for this exact same reason a few days ago, pretty much word for word. We've been troubleshooting this issue but are unable to reproduce it in our environment and are out of ideas on what the issue could be. All of our agreements, tax forms, and contacts on App Store Connect are up to date.