Fixed!

This is true! There was indeed still an issue. I wrote some additional tests to prove this case and re-worked the solution to meet these tests:

The previous tests confirmed that the row

4|#|2|2

when shifted right, would combine correctly to

#|#|4|4

But the solution still combined the row

2|2|#|4

incorrectly into

#|#|#|8

This has been fixed. Please re-try your solution to confirm, and thanks for the feedback!

Thank you for your report! You were correct, there was an issue with the test case that has now been resolved. Thank you for your example. It was very helpful for debugging purposes.