It's more like your traktor's tanks can't swallow the amount of oil coming back from the splitter.
The tractor's hydraulics are not involved. It's a stand-alone spitter with it's own hydraulics. Here's a photo:

The tank is the large box that's also the axle. The vent is way on the left side, so if that side is low it can overflow when there's still plenty of room in the tank because the air is trapped on the high side of the long tank. Not a great design, but cost-effective I suppose.
My surprise is mine seems to overflow only when the ram is in the vertical position, like in this photo, and only when the oil is hot. It seems that more oil flows back to the tank in this position then when it's in the normal horizontal position for some reason.