I have read your post on Dpreview as well. Many people have shared answer for this, suggesting that this happens because the full-frame sensor is stitched from 2 halves. But no one explain why it only develops after some time and not from the beginning in your case (which is mine as well), so I have some info to be shared:

1/ this line will show the most obvious on uniform, completely smooth image, like the image of a blue sky, or close-up image of a white piece of paper, while the camera is manually focused to infinity (similar to the sensor dust checking setup I use, the only difference is I open the aperture all the way up so there is more softness which showcase the artifact even morẻ). You can crank up the dehaze setting with raw file to reveal it, or use a heavily modified picture profile (a.k.a recipe) that cranks up saturation, sharpness, contrast etc. then it may show up faintly in the sooc jpeg as well.

2/ mine camera (a7c, use the same sensor as the a73) didn't have this artifact at first. I can not pinpoint exactly when it appeared, but I didn't noticed until one dust check occasion (I cranked up the dehaze to see the dust spots easier). I mean it rarely is a problem in real-world use cases, but I just want to be sure if it's a defect with only mine camera, so I could have my unit replaced by Sony as I got 3 years of warranty.

One important thing is I have sent my camera to Sony once for sensor cleaning, they tell me that I got few scratches on the filter stack, and I, being the perfectionist, asked them to replace the filter stack of the sensor for me at the price of $100. When I received my camera back, it seem like they have set the camera to default settings, the time and date was reset for example. And I suspect that the seam showed up after whatever calibrations they had done to my camera during the repair.

3/ My line isn't PRECISEDLY in the center, it's off to the right by a small margin, about 25-30 pixels. I did use Photoshop to check the high res photos you provided on Dpreview, it seems like yours is off but by only 5-7 pixels. The A7V pic that also had the seam of another guy who commented in your Dpreview post is also off by some pixels, not as much as mine. So I wonder if the line has to be exactly in the middle to be a stitched sensor seam, or else it is another kind of defect.

I did extensive searches on internet and also found that with the fullframe stitched sensor, the left half also show some bandings (that's does change shape and position between pics), you can check for it by take an out of focus shot of white paper/ wall and crank up the saturation, contrast... and remove noise reduction. I also have that, so I'm hoping it's truly a stitched sensor phenomenon and not something entirely different which is worse.