Quick update, I took these two photos of the sky this morning focusing as close to the camera as possible to get a good image of the dust on the sensor.
Left = ISO 200 125mm f/36 1/20sec
Right = ISO 25600 80mm f/36 1/8000sec
View attachment 88475View attachment 88476
As you can see, it's very faint and diffuse at ISO 200 (so faint you might not be able to see it in this image) and then very sharp at ISO 25600 even though the aperture remained the same in both photos.
I see that you also changed the focal length between the two pictures. You made at least two changes, and are attributing the change in visibility of the defect to one of these changes.
It seems to me much more likely that changing the focal length would affect this than changing the sensor ISO. The ISO setting doesn't affect the optics, after all.
Try going back to ISO 200, with the lens zoomed back to 80mm, and the aperture at ƒ36. I bet the defect will show up just as it does in the ISO 25600 picture here.
Of course, this whole exercise, at this point, really isn't important. What matters is that we know there is something in your camera that isn't supposed to be there. I'd have to say that it was almost certainly a bit of hair or fiber stuck to the front of your OLPF, but you report that you had your sensor (actually, it would be the OLPF) professionally cleaned a month ago, and also that you can find this same defect in pictures taken well before as well as well after this cleaning. It seems very unlikely to me that anything even vaguely resembling a competent attempt at sensor-cleaning would fail to remove such a hair or fiber.
It's also been suggested that perhaps the contamination is between the OLPF and the sensor, but it's also been adequately explained in this thread why this is extremely unlikely. And it's difficult to imagine how a scratch or crack could have come to be there, unless by a very clumsy and incompetent attempt at cleaning the OLPF, or by a defect that was there from the factory.
I think I am left thinking that the most likely explanation, albeit not one that in itself seems terribly likely, is that the “professional” who allegedly did the sensor cleaning did such an unbelievably-poor job of it that a piece of hair or fiber that was there before the cleaning, and ought to have very easily been removed, remained there afterward.
My recommendation, to begin with, is that you need
this:
Don't settle for any lesser brand or model. The large-size Giottos Rocket Air Blaster. It has a special valve to keep it from sucking in dust and blowing it back out on to whatever you're trying to clean. With your lens off, and your mirror “locked up for cleaning”(which also opens the shutter, exposing the OLPF), hold the camera face-down, and use this blower to blow air forcefully against the OLPF. This will dislodge most dust, and it would very likely dislodge the piece of hair or fiber that would likely be causing the defect you're seeing in your pictures.
If that doesn't get it, then the next thing I recommend is
this brush.
It's the same basic concept as the “Arctic Butterfly” of which you may have heard, but this is simpler, and much less expensive, and works at least as well. You “charge” it by blowing air through it with your Rocket Blaster; this does two things. It blows off any dirt or dust that may currently be on the brush, and it creates an electrostatic charge that causes the brush to attract dust that it encounters on your OLPF.
Normally, I'd say that if you still have contamination problems after using this brush, then it's time for a wet cleaning; but in your case, if what is causing your issue doesn't come off from using this brush, then it's time to start seriously considering the possibility that either it's between the OLPF and the sensor, or else it's a scratch/crack; either of which would put it well beyond a DIY remedy.
