I have heard that there is some truth to this myth. One of the debates arises from the extremely complicated question of Schleswig-Holstein, two provinces that belonged to Denmark but were members of the German Confederation. When the Danish king tried in 1864 to bring these two provinces into a more centralized Danish state, Bismarck used this as a pretext for war. However, Bismarck enlisted Austria to help Prussia fight a short war against the Danish. After defeating the Danish, Austria received part of the territory and Prussia another part. Austria's part was then forcebly taken over in the Austro-Prussian war of 1866. Bismarck's reason for fighting the Austrians was to take the territory they had taken from the Danish, which he did. What historians still debate is his reasoning for bringing Austria into the fight in the first place. His military didn't need the support, so why would he allow the Austria's to join him and receive some of the territory if Prussia was gonna kick them out eventually anyway? Bismarck's decision to do that is brought into question a lot.

Incidentally, historians always find points to debate in a given period. The fact they disagree on Bismarck says as much about those studying him as it does about the man.

This is because there is a strong emphasis on originality of approach as a good quality in new historical work. Finding new source material often isn't likely, but it's always possible to re-evaluate what you've got. That gives a pattern of very reactionary literature on all sorts of topics, and Bismarck is just one example. Historians swing to extremes on a given topic so that they're heard, and while sometimes differences can be reconciled it's not always practical.

I can give examples from histories of specific periods if interested.