In all PS versions 23.0.2 and earlier setting Grid to 33.34 Percent with Subdivisions = 1 created a nine-section, tic-tac-toe-style grid which divided both horizontal and vertical axes into three equal sections, providing nine working sections, each having a ratio equal to that of the canvas.
In PS 23.1 setting Grid to 33.34 Percent with Subdivisions = 1 creates canvas full of squares, each of which has all sides set to (essentially) one third of the horizontal value of the canvas only, regardless of vertical value. There appears to be no setting available to force behavior to divide vertical values by any percentage.
If this change was purposeful, it has deprecated one of my most important tools for composition. If it wasn't, please arrange to align this setting with results obtained from previous versions.
PS 23.1 uninstalled; reverting to 23.0.2 pending correction or confirmation of intent to change.