If this is accurate (and DGIndex is not always accurate) then this is not ideal. It doesn't look like there is a way to turn off de-interlace in ConvertXtoDVD based on the manual. You might try a different program. I use an older Nero but I do NTSC instead of PAL. A quick search brought up this free one:

http://www.videohelp.com/tools/DVD_Flick

I don't know enough about PAL to tell you why a conversion to progressive would play incorrectly as interlaced. If your content started out as true interlaced content (which yours is) then there is no good reason to make it progressive.