Hi Elona,
Welcome to Microsoft Community.
I'm Hahn and I'm here to help you with your concern.
This may be expected behavior based on how the Surface Book charges one battery and then the other. The batteries in Surface Book may charge at different rates, depending on the power state of each battery. Whichever battery has less than 20% charge will be charged first. When both batteries are at 20%, the clipboard will charge to 85%, and then both batteries will charge to 100%.
If one of your Surface Book 3's clipboard or keyboard has been charged to more than 20%, but the other has not been charged, there may be a hardware problem with your Surface Book. How to get service or repair for Surface - Microsoft Support
I hope this helps. If there is anything not clear, please do not hesitate to let me know.
Your Sincerely
Hahn. W - MSFT | Microsoft Community Support Specialist