Rules error

After asking a couple of questions on the datascript slack, @tonsky asked me to make a gist with my error in it. The function (connected x y) should return a set of all diagonal neighbors of neighbors on a cartesian grid but in cases where the (= x y) it ignores neighbors that are also (= x y).

The gist is at https://gist.github.com/Tavistock/6305de1b890e3a60489544ae139b102f. for background this is for implementing a game of checkers or draughts.

This image shows the degenerate case where the orange highlighted checker is selected and the green squares are it's connected. Notice that (= x y) for the checker.

This image shows the correct highlighting when selecting a different piece and using connected. Notice that (!= x y) for the checker.

My question is whether there is an error in my code and reasoning, or if this is a problem with Datascript.

Start with the linked gist and the (connected x y) function, then reproduce the Cartesian-grid case where (= x y) omits neighbors that also satisfy that condition. Compare it with the non-equal case and determine whether the behavior comes from the reported code or Datascript. Done means identifying the cause and confirming the expected diagonal-neighbor set.