Some ArcGIS Earth 1.10 users who use PKI authentication within KML services may be prompted for a username and password instead of a PKI certification window. A fix for this is included in 1.10.1 and users can download and install the latest release here.
For more details see the KB article created to support users who have this issue, see
https://support.esri.com/en/Technical-Article/000023245