This might be a bit outside what you are looking for, but I think it is obvious that we can come up with some cases where it would be ok for a historian to lie. To take a silly example, if I know for a fact that 3 million people will be shot if I publish a paper on George Washington, I think most would agree I ought not to publish. My contribution to Washington scholarship probably isn't worth the cost of 3 million lives, after all.

Of course, this sort of example is ridiculous, and really only of interest if you are engaged in purely speculative ethical theorizing. In the real world, this highly artificial scenario is just not the sort of situation we ever find ourselves in. In point of fact, I can't think of any real world example in which it is even plausible that a historian does not have a moral obligation to tell the truth. Do you have any such examples in mind?

It is broadly speaking a Historians job to tell the truth

The origins of the modern study of history are sort of embedded within this idea. Historicism (von Ranke and co.) placed great emphasis on building an accurate record of the past through primary source analysis, and these sorts of practices exist to this day amongst trained historians. Before the 18th/19th century many histories would have other motiviations, and also have the interests of patrons in mind, this of course did/has not died entirely but the historicist idea of the professionalisation of history has introduced certain unofficial standards, and now through things like peer review blatant lies are met with equally blatant rebuttles.

The historicists believed that they had to tell an impartial and accurate narrative, however there was always slant it is simply unavoidable. Now we write history knowing our own biases, in fact we are all biased and write with opinions because we believe those opinions to be true, however those arguments must be supported by the evidence presented through both primary and the arugments of secondary sources. If history was just a string of facts there'd be no need for historians at all.

I suppose the emphasis is on trained historians. Seemingly anyone can publish a history book and call themselves a historian, but they aren't really a historian. In the same way I wrote a book on science, I wouldn't automatically become a scientist.

To be a historian you must be skeptical, beware of taking anything at its face value, analyse and cross reference, to build up a narrative and argument which addresses whatever issue is at hand. Though we don't believe that history is a science in the same way the historicists understood it, we still understand that sources must be analysed and scrutinized, arguments must be evaluated and challenged.

what I am saying is history is more than just a list of historical facts or truths. It is a series of arguments with evidence. What is true or not is often open to interpretation, and for this reason you should never really take anything at face value.