[Kansas Ref]

The T appeared to be totally occupied with the matchup right in front of her, as she can be seen administering an intermittent "5 sec closely guarded count" on the ball handler. Can she see in her periphery that an IS had occurred? Should she have been expected to be accountable?
I agree with the posters who have asserted that the L needed to expand/come off his monitored area, which he did judging by where his gaze seemed to be. Let us assume that he saw the IS, why would he not kill the play? I would presume that he did not want to appear to be "fishing in someone else's pond". But he was ball-watching.
Cameron I am enlightened by your comment " PCA is about one-ball coverage ".
JRut I am enlightened by your comment on " following players moving from your primary area"; this concept is consistent with what we do in following a dribbler from our primary area when they drive to the basket away from us.
BobJenk. I concur with your assertion that the T should've seen this IS ; an experienced ref should be able to "chew gum and walk simultaneously".
Raymond's point of placing responsibility on the L to come get that foul is certainly not beyond what should be expected from a mindfully-engaged ref.
Billy, thanks for posting that vid!