Hello SS,
I'm not suggesting that your problem is due to coma or astigmatism, but I would suggest you download this Lens abberation test page (scroll down). It's like a whole lot of evenly spaced stars and then try to replicate your situation to understand the issue more....
https://www.lonelyspeck.com/a-practi...erration-test/
All I can think of is that somehow the lens optical axis and a line perpendicular to the sensor surface are not parallel and hence one side of the image plane is closer to focus than the other. If so it should be more obvious at larger apertures, so comapre f1.8 with f16. Is there any sign of play or excess wear in the lens mount that might be causing this? With the lens mounted on the camera grab the lens barrel and try to push it from side to side. Any history of any loose lens elements or lens dropped at any time or repairs?
Best
JA