Thanks for letting us know about this, and I apologize for the delayed response. I'll pass the issue detecting your device's CPU along to my team and see if we can resolve it in a future update. Could you let me know the model number of your device, to help us investigate the issue more quickly?
Regarding the older versions of Geekbench, if you have purchased Geekbench 3 or Geekbench 4 through the Google Play store, you should be able to download them again through the store's Account menu under Purchase History. Would that allow you to test your devices that aren't supported in Geekbench 5?