You may just need to update it once you change the property, or regen the drawing. I think a nil is always returned.
I tested your code on one of my dynamic blocks, and it changed the property, and it showed it right away. And my property looked like yours. The only thing I can guess is maybe that is the wrong property. Try changing one of the others, or looking at the block in the block editor, and make sure that is the right one. Besides that, I'm not sure what else to try, as I don't use dy blocks.