I planted a kerria japonica in a large tub last year. Part soil and part compost. At the moment it has many flowers but only a handful of leaves.
I live in the North West of England. Shrub is in sun from noon onwards. Where are the leaves???
The most obvious problem may be the vessel in which the plant is situated. If there is insufficient drainage, the leaves could be falling off due to insufficient root respiration (drowning). The leaves would then yellow and drop off. You mention that the plant has full sun from noon onwards and that could also be part of the problem. Kerria japonica ‘Golden Guinea’ prefer to be in shade, more to keep the flowers from fading but it could also be affecting (burning) the leaves. Try relocating the plant to a more suitable location and see if that helps.
After looking at articles on the internet, you might come across some pictures of Kerria japonica that have flowers on almost leafless stems. So perhaps that is the growth habit of your particular cultivar. Plants often flower before their leaves come out in full. Here is a link with pictures that show the plant in a similar state as you describe.
Apart from that, there is a possibility that the plant could be diseased and we have a published article that mentions this problem on another cultivar (K. j. ‘Pleniflora’).