Hi Maya and welcome.
Bleached flour is treated with one of several chemical agents to whiten it and to rough up the surface of the flour particles. It is available in the U.S. as either cake flour (about 8% protein) or All-purpose flour (around 10-11% protein). American cake recipes that call for softened butter need bleached flour to keep the butter in suspension. Sponge-type cakes and cakes made with oil do not necessarily need bleached flour. In many countries outside the U.S., bleached flour is not available to the home baker. There are two ways to work around this, first to heat-treat flour to mimic the effects of bleaching. Search over on the blog for “Kate flour” for details of this method. The second approach is to cut the flour with potato starch and adjust the leavening. This approach can also be found over on the blog, search for “Power of Flour”, there are four parts, I believe.
As for the cake sinking in the middle, that could be a result of your unbleached flour. It could also be a sign of underbaking, either by time or temp (oven too cool or not long enough baking time). Or it could be from too little flour or from too much butter or sugar, if your weights were off.
