Since the ESXi version 6.5 Update2/6.7, you may see some of hardware sensor may show as Unknown.
VMware accept this issue as part of KB https://kb.vmware.com/kb/57171 and mentioned it as expected behavior.
To give more clarification, the sensor status presented in version prior to 6.5 U2/6.7 used to show green for any status that fell under the list category that could not be read OR understand. It used to show green for anything that is actually unknown. This behavior got changed in 6.5 U2 and 6.7 to show as unknown wherever the values cannot be read.
You can contact your hardware vendor to know why ESXi unable to read certain sensor that showing as unknown.
This behavior is because of defect fix as part of KB
VMware team explained the changes to Hardware Health Monitoring in vSphere Stack https://videos.vmworld.com/global/2018/videoplayer/23425