I always ask myself if you can expect much better in this kind of situation, regardless of the lens:
The scene contrast is really low, plus the total number of photons is also low (ISO12800 is pretty low exposure). The resolution seems decent, as you can see individual plumes sticking out, but the contrast is not sufficient to show the structure of the plumage on the breast for instance. In-camera noise reduction was probably playing its part and further smoothed out the little detail that was left, detail which is of high frequency and therefore hard to distinguish from noise.
The fact that the tree bark has much higher contrast at small spacial frequencies and therefore does not interfere as much with the noise subjectively adds to the soft impression of the bird.
It is hard to see whether the AF is mis-tuned using this kind of test. On the other hand, we cannot be sure that AF tests and adjustments done in high-contrast light extend to low-contrast situations. We hope they do, but low contrast combined with non-flat targets tend to fool the AF quite a bit.
I think for optimal operation of the AF, you need a little bit more contrast (a bit of directional light) and this will also make the image pop more. It is hard to make a pleasing image in this adverse light unless you somehow manage to turn it to your advantage.