Recently added subitem column has disappeared

I’m baffled… Earlier this week I inserted a Subitem Date Column titled “Subitem Initiated” in order to track the age of subitems through the life of any given project on the Board.

That was Monday. I used it as recently as yesterday (Tuesday) evening. Now Wednesday morning it is gone, with no mention of it in the Activities Log.

What is going on here?!? So frustrating…