With <RequiredRuntime Name="CSXS" Version="8.0" />, extension fails to load.

I am a beginner in HTML extension development.

I am using Photoshop CC 2018 and if I set the RequiredRuntime to CSXS version 8.0 in the manifest.xml, the extension fails to load with error log "ExtensionManifest is not valid for this CSXS runtime". The extension loads fine when I change the Version to 7.0.

I believe I have the CSXS runtime 8.0 as csInterface.getCurrentApiVersion() returns {"minor":0,"micro":0,"major":8}