Didn't this type of play happen one other time this season for a different team? I hadn't seen it in a while until this season. I recall learning about it in a game Dallas played against the Cardinals in Phoenix. David Boston came down, his toes hit first and then his heel landed out of bounds, so it wasn't a TD. I don't think it a great rule, but it is easy to see and was the right call on the Lamb play.
I look at it has the receiver demonstrating control of his body and a continuation of a movement. With a toe tap or a toe drag, it indicates the receiver controlled their movement within the bounds of play. You don't need a heel to land to count as catch in either instance. The heel coming down is a continuation of the feet coming down with the toes leading the way. If the heel stays in bounds the receiver again demonstrates body control and when it lands out of bounds they don't. Sucks it went against Dallas, but maybe in several years, the call will go for Dallas. It is a rather rare occurrence.