I haven't tried to use mine at long distances yet, other than to treetops. But I will say that from the experience of using other lenses sharpness at a distance is always seemingly reduced. It's something I've always called pixel per eyeball. In this instance, perhaps pixel per feather. ? If you shoot closer, and a bird is basically the whole frame, there are a lot of pixels to give resolution and detail to the feathers. If you shoot farther away, then a lot of those pixels are going to everything else in the frame - lake, trees, sky, etc. It's the same with portraits. A headshot will always appear sharper than a full body image.
Then of course there are the non-lens factors, like camera resolution, technique, shutter speed, etc. A Z7 will be much sharper than a Z50 with the exact same specifications because there are more pixels and you can zoom in further (crop factor of the Z50 not withstanding).
All that said, I do have a friend who also recently got this same lens and she is unable to get anything sharp, even shooting still subjects on a tripod. She has had it to the camera shop once and they confirmed then tried to calibrate with the dock and she is still unhappy, so she is going to exchange it for a different copy. I know several people in my current wildlife workshop are all using this lens with great results, but there can be bad copies of any lens.
On the other hand, if you are only seeing issues with shooting at a distance, I think it's just a matter of tweaking your technique a bit. Do you have a monopod? That could help. There are also focus limiting buttons on the side of the lens. Have you checked those? All my other lenses that have that only cut out the near focusing, but the Sigma allows you to cut out far focus. I would check to make sure you are set to infinity or the far focus option.
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If you are really concerned/curious you could post a couple of samples of what you are getting and your settings. I try not to shoot below 1/1000 and then I also shoot in burst mode just to be safe.