Seeing how he and his tricorder have completely forgotten Klingon anatomy at the most inopportune of times, I don't find this the least bit surprising.
Hmm. As far as I know, Bones never treated a Klingon onscreen before this movie. And his tricorder only scanned Arne Darvin, who had supposedly been altered somehow to pass superficial muster.
"Errand of Mercy": no Bones action with Klingons.
"Trouble with Tribbles": only the Darvin scan.
"Friday's Child": no opportunity to study the Klingon.
"A Private Little War": no opportunity to study the Klingon.
"Day of the Dove": Bones busy with his own crew, but everybody rendered immortal and confused anyway; not a great learning opportunity.
ST:TMP: no opportunity to study Klingons.
ST3: no studying or treating of Klingons, and his marbles were pouched elsewhere anyway.
ST5: no studying or treating of Klingons that we could see, unless there were some hangover cases or STDs or whatnot after the big party.