Even with Apple products, the Bluetooth menu isn't consistently showing me the battery percentage. With the latest Big Sur, the BT menu won't show me the battery percentage on my Magic Trackpad unless I enter the Bluetooth Preferences panel first. Then the percentage shows up. The BT menu doesn't show the battery percentage on my Logitech BT mouse at all. I have to run a Logitech application to get it to show me the battery level on that mouse. It doesn't surprise me that you don't get a battery percentage of another third-party mouse.