Not sure about this part, but it appears the antialiasing of the shadowmaps is based on a percentage of the total map size rather than a specific number of pixels.

That would explain why, if the field of view is really wide and the shadow casting object very tiny in that view, that increasing the size of the map will never produce a shadow.

For example, if the antialiasing is 3% of the total map size in pixels, and the shadow casting object occupies less than 3% of the field of view, it's shadow will always be antialiased out regardless of map size.