panorama – Why is Photoshop’s Photomerge stitch having problems correcting exposure on huge amount of photos?

I have a problem. See photo 1 (with all stitched) and photo 2 with only top left. You see in the smaller it’s OK. In the bigger it didn’t correct any small difference in exposure (due to sun going behind and out of clouds). All images were shot M mode with same settings. Developed with same settings. This happens with pano’s with MANY photos.

The bigger image was stitched from 80 photos. The smaller was stitched from 20 photos. But the thing is, I have also done stitches with 120 photos which has worked. Sometimes it doesn’t, which is strange.

Any idea about this? As part by part works this must be some limitation? memory?

Using fewer source images is not a solution — I have my reasons to use this many.

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