I got it. I ended up having to draw out the spline and extrude it, then boolean it

I've had issues in the past where I was extruding text, then Boolean diff with a solid. Most letters would subtract, but not all.

Close examination showed that the letters that failed had points bunched up together at corners of the text outline.

Running the RebuildCurve script to simplify the text outline before the extrude solved the issue.

Ed