I looked in the filesystem and the file created by importing the webpage looks like what it should be. So maybe it is a display problem in the Windows Beta?
Not necessarily. But without support for webarchive format, Scrivener can’t do it.
Safari (for Windows) is able to read those, and then you can save to some other format that Scrivener CAN read. Or drag and drop from Safari the bits you want (warning; dragging/dropping web content may still be a bit buggy).
There’s an app for Google Chrome that purports to be able to open them and save them as zipped HTML.
Microsoft Word can open them and save them to some other format.
There may be other software that can read them and write to a different format, but those are three that came up quickly in a Google search. I checked that Safari for Windows can read them, and it can.