Journalists receive leaked information all the time, and do not always run with it.
The most important consideration is probably whether the information is partial or tells the whole story. For example, many media organizations received copies of the Trump dossier — the one with the Russian golden shower story — but chose not to run it because they didn’t have complete information. Buzzfeed, however, decided to reject that standard and publish.
In this case, one tax return from one year is clearly incomplete information. At best, it gives a partial picture; at worst, it’s misleading. Maybe the 2005 return looks similar to all the others, and is therefore a good illustrative example. Maybe it’s an outlier, and therefore presents an inaccurate portrayal of Trump’s taxes. Both are possible, but without more information, we can’t know which is right.
Journalistic ethics does not require publicizing every piece of partial information. Quite the opposite.