I ended up reworking the blueprint for the text field with the inner shadow. Now, the border and shadow calculations are a lot more deterministic.
Here’s an example of the textbox material with image size 1024x1024, margin 0.5, 0.075 (NOTE: The horizontal and vertical margins are different), box width 0.96, box height 0.96, and corner ratio 0.65 (pictured here with shadow offset X of 2.5, shadow offset Y of 5.0, shadow scale of 1.5, and shadow sharpness of 0.0):
The blueprint link is the same as before: