23.976 FPS Sources are based on 35mm FILM and therefore they can't be interlaced this is also known as progressive.
So you still have to encode at 23.976 (incl. NTSC Resolution set in Gripfit or calculated using moviestacker) and also set in the encoder the "3:2 Pulldown" option! .... try it.
Then import your 23.976 pulldowned stream into your DVD authoring Application. And thats they way how this works logically.
Btw: If you check your stream after encoding, the PC will say that this stream after encoding is a 29.976!
Tricky, but in real its a 23.976 with 2:3 Pulldown. Many progressive (Blockbuster Movies) DVDs are encoded using the 3:2 Pulldown.
Well in this case as said a 23.976 isn't interlaced ...
BUT generally only trust your eyes! This means if you see the source video on a pc screen and there do appear Comb artefacts in fast moving scenes therefore its in an interlaced condition, also known as FieldBased. Progressive = Framebased