You’re running into 2 issues. One is the barrel distortion associated with ultra wide angle lenses.
https://en.wikipedia.org/wiki/Distortion_(optics)
The second issue is Ultra wide angle lenses often appear less sharp at the corners because of the inherent design challenges in capturing a very wide field of view, which can lead to issues like light falloff (Vignetting), barrel distortion, and field curvature*, causing the image quality to degrade significantly towards the edges of the frame, especially when compared to the center sharpness; this is particularly noticeable on lower-end lenses or when shooting at wide apertures.
“Curvature of field" in an ultra-wide angle lens refers to an optical aberration where the plane of focus is not flat, meaning the image edges appear blurry while the center remains sharp, which is a common issue with wide angle lenses, especially when the focal length is very short (ultra wide angle), causing the image to appear slightly curved at the edges; this is a significant concern in photography as it can affect image quality, particularly in the corners of the frame.
https://petapixel.com/2016/12/21/field-curvature-tricky-problem-photography/