Large points that extend across the dateline when viewing at low zoom levels are being clipped. The points do not fall exactly on the dateline, but when scaled to cross the dateline, they are clipped.
This is being seen using Tilemill 0.5 on OSX. See here for example of clipped points.
To get this working will be pretty tricky and likely require rendering the data twice. It is doable, especially with the help of dateline master @rcoup, but not on the list atm to tackle.