If you connect your Windows SSD to slot M.2B-SB and everything work fine than it is either the CPU or Motherboard issue for slot M.2A_CPU. But if the same thing happens in slot M.2B_SB than it sounds like the SSD issue.
If you connect your Windows SSD to slot M.2B-SB and everything work fine than it is either the CPU or Motherboard issue for slot M.2A_CPU. But if the same thing happens in slot M.2B_SB than it sounds like the SSD issue.