Chiffon cake constantly has an unfilled bottom

This will happen if you grease your cake pan or if the cake isn't allowed to fully cool before being removed from the pan. Another possible culprit is overbeating your egg whites.

Egg foam cakes, like chiffon, blow up with steam as they bake. The tube pan gives them more surface area both so they bake in the middle and also to give them more to cling to.

After they bake the cake is inverted until the egg proteins set. If not inverted, the cake will collapse as the steam escapes. A good rule of thumb is about an hour rest but the cake should be nearly room temperature.

If the tube pan is greased then the cake will not adhere near the end of cooking or through the inversion rest period.

The consistency of the egg whites is also important. If the whites are overbeaten they will lose some of their elasticity and give up their steam when they are stretched too far. Chiffon cake recipe do call for firm peaks, that is if you pull your beaters out the resultant peaks don't droop. But if you beat much past that phase the whites can get fragile.

Keep in mind that you should expect the cake to shrink slightly from it's max height when the steam is hot, but nothing like the collapse in your photo.

I don't believe that over or under beating the egg yolks would have caused the behavior you were seeing.