The map contains the file scripts/omaha.shader. However other mappers also have used the same filename for their shaders in omaha.pk3 and 1944_omaha_b3 (and earlier versions). Consequently these can be adversely affected graphically since the last shader file (of this name) that is loaded dominates. So omaha.pk3 if present in /etmain would dominate the other two. Or if only bloody_omaha and 1944_omaha were in /etmain then bloody_omaha would dominate (and make 1944_omaha_b3 looked crappy and unfinished).
However merging the various shaders in these files seems to create a workaround.
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The shader
z_omaha_fix1.pk3 workaround is now tested and available on my filefront
page.
Or grab the attachment here and rename it as .pk3