If you have a TableMatrix with the SquareCells=true attribute and the array has a width of 1 it will render with non-square cells.
[TableMatrix(HorizontalTitle = "Blocks", SquareCells = true, ResizableColumns = false)] public GameObject[,] BlocksMatrix = new GameObject[1, 3];
Unity Version: 2018.1.0b13
Odin Version: 22.214.171.124 (This version isn't in your drop down list)
OS: Win10 16299
Editor Only mode: I don't know what this means. I did not supply any extra attributes to the C# script like execute in editor mode.