Currently, Microsoft Edge generates HAR files with a malformed HTTP version string, e.g.
When Fiddler's HAR Import feature attempts to parse this data, it fails and this results in omitting *all* of the headers from display.
To mitigate this, Fiddler should look at the httpVersion string and if it lacks a slash, it should either add the slash (if this is enough to placate the response parser) or it should generate a dummy value for the entire token (e.g. "http/0.0") so that the rest of the header values are parsed correctly.
(I have filed a bug against Edge too).