Background char color in header on PDF export

If I set some text in the header to have a background character color of say Yellow, it looks fine in print preview, but when I export it to PDF, the color is missing. Is it that I am doing something wrong with the PDFExport settings, a bug in PDF export, old Acrobat reader, or is this just the way it is for PDF export?

Using WPTools 5.17.4, WPDF 2.51, Delphi 7

I updated to 5.18.5, but while the PDF problem has been fixed, the behavior of the background color is now different. In pre-5.18.5, when I set the background char color of a word in the header, the color height was the character height... desired. But now, the color height is the line height... not desired.

For example, my header has an image in it, causing the text on the same line to be pushed down to line up with the bottom of the image (see my other post about an image alignment problem with header images). Prior to the 5.18.5 update, the background color would be only as high as the characters. But now, the color stretches up to the top of the header area by the amount the image had pushed the text down. Understand??

Eric