You mix up HDR Recording and Dual ISO recording.
HDR Recording alternates between 100 and 1600 every other frame e.g. the flickering you are seeing. And needs to be merged/blended. Search the forum for post process of HDR.
Dual ISO records 100/1600 ISO simultaneously (every frame), but every other Pixel Row is 100,1600,100,1600 etc. And also has to be merged with Dual ISO Post Processing.