What camera? These could be compression artefacts (increase your jpeg quality), or could be caused by lens correction. Turn off lens correction. For some cameras (mirrorless cameras such as Z7, etc.) in Lightroom and in ACR you cannot turn off lens correction and thiss can cause these curved artefacts
It's a Nikon D850, the export quality was already 100%, it's a 42,8MB file.
I have turned off the Lens correction and the problem persists (image below), although it seems less apparent and mostly lower in the image. Might also be less visible due to the fact that the Lens correction reduces the brightness in the lower corners.
It's possible that some of the artefacts are real and present in the original data. This sometimes happens to me in deep blue skies and is caused by the red and/or green channels being very low. It can also be a color management issue where if you have an 8-bit file and are using an 8-bit display with another 8-bit display color profile, you can get slight bit errors that can cause posterization. That's something you probably cannot get rid of when using jpeg files no matter how high you set the quality.