In the case of Brexit, the political news media was correctly reporting that the British public preferred Remain until a sudden swing in the opinions of undecided voters in the few weeks before the referendum.[0]

In the case of the US, the political news media was correctly reporting that the American public preferred Clinton to Trump.

Neither case should have damaged their perceived credibility, except in the minds of people who believed that there was a conspiracy of under-reporting the popularity of Brexit and/or Trump.

[0] https://en.wikipedia.org/wiki/File:UK_EU_referendum_polling....