could be anything.. a bit too much moist upon transplant or lighting hitting on one side only, or sun too hot through the window, but coldest spot for it at night, there is too much of cold and hot, rather than a constant temp. that tells the plant to survive..and it drops its leaves to sustain the roots. do not add more water, if it feels moist to the touch.
try bringing in a bit in from the glass. if you can when sun is not too hot bring it closer to light, but back it away from cold nights and scorting sun. Even in the fields, young trees drop their leaves. it is a delicate stage. Sometimes fertilizers make them grow, but their roots cannot sustain the growth, there they also drop their leaves. My Black Madeira is inside of a tall cold frame inside of a 5 gallon water bottle with no bottom. So far it is just a stick with a tiny green eye. I love it. We had ONE warm say meaning with sunshine and it got hot in there almost 80, for that I dropped a shade fabric over the bottle. I know the sun will scortch the new leaves.
Please who grow kiwis also have that ... figs, kiwis and many others are sensitive. Unlike 'weeds' we don't want.