And how do you define "backwards compatible"?

How can you be sure that something is "backwards compatible"?

By running tests. And as it happens, the original coreutils did not have a test for this particular edge case.

Now that a divergence of behavior has been observed, all parties -- the coreutils devs and the uutils devs -- have agreed that this is an unacceptable regression and created new test cases to prevent the same misbehavior from happening again.