Problem exporting poster design for print

Apparently you have a lot of noise in this file. That is the root of this.

You absolutely must view this at 100% to get a correct preview of layer blending and adjustments. 100% has nothing to do with size. It means one image pixel is represented by exactly one physical screen pixel.

Whenever something seems to change when merging layers, it's because you are not viewing at 100%. The merged result is correct. The preview is misleading and incorrect.

For performance reasons, all blending and adjustment previews are calculated on the on-screen version of the image. When you are zoomed out, that means a resampled and softened version of the image. Pixel levels are averaged out. You get a lot of intermediate values that aren't there in the full original data. For the same reason, don't resample for output without checking before and after at 100%.

When you merge, commit an adjustment etc, the numbers are re-calculated on the full original data, pixel for pixel.

Viewing at 100% avoids all this. You see every pixel before and after, and so nothing changes.