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Thanks for all input!
I was thinking field curvature too when I started testing, but there is only a very slight refocus necessary when moving focus from centre to corner in liveview so in my opinion this is only part of the explanation.
My testing: tripod, focus in liveview, flash in combination with existing light, there is some difference in lighting between different apertures, (and slightly unevenly lit test setup).
PP: Adjusted exposure and WB to match the shots. Automatic CA removal in Lightroom, no distortion correction, 50% correction of vignetting in LR.
Test scene, Distagon 15mm f/5,6:
1:1 crops- centre/lower right corner Distagon 2.8/15mm, Corner Distagon 2.8/21mm, Corner Canon TS-E 24mm 3.5 II