Attached is what our engineer sent back to me after I submitted your question to him.
After reading this it sure looks familar to me, so I looked in FEMA 350 3.3.2.3 and it almost mirrors this text.
Does this answer your specific question about whether it is prohibited by D1.1 or other code to weld in the "k" region of a WF beam...I don't think it clearly says either way in black and white, it simply recommends against the practice in certain conditions.
As far as if you are inspecting and run up on something like you did, and have a concern or a question about your findings...
... I would simply note your concern in your report and that you didn't see any code violations against this practice that you are aware of, then cite this article as your basis for your concern and let the EOR make a decision whether to accept or reject.
just my two cents