The hover in grid has to be on entire row/item, not only in frozen/normal side, as it is now. Office 365 & Nova theme have default support for item hover, but this does not work properly when having frozen columns.
The reason for this behavior is that when using frozen columns the grid is rendered with two separate tables - one for the frozen columns part, and one for the rest. Unfortunately there's no way to support by default row item hover in both of these tables simultaneously, unless using custom hover implementation. Feel free to integrate your own custom css hover if applicable, Sergiu.