Exact same issue here: Magic Mouse 1 will not scroll on the iPad Pro 2020 12.9" (current version: 13.4.1). I went on Apple's chat support and they're saying it might be a glitch in the software, so they're going to try to update the iOS version on the iPad and hope that it will resolve this issue. They also said that if the issue persists after the update, then the next option would be to have the iPad and mouse inspected by one of their technicians. Now, because lots of people are having the same issue, I don't think a technician will figure the issue out by looking at the equipment we already own - the issue is with everyone, not just a particular device. The assistant I chatted with also sent a message to www.apple.com/feedback to report this issue. Fingers crossed they will sort this out for us and we can carry on using the Magic Mouse 1. I would recommend for more people to report this issue to Apple. The more the merrier I think!