Transparent pattern tiling in Image Magick generates broken pattern - imagemagick

Question

I am trying to generate image with background and pattern like in this image.
Currently I have this command
magick convert \
\( \
-size 512x512 \
-define "gradient:vector=0,0 0,720" "gradient:#5DA2C2-#C70AA0" \
\) \
\( \
-size 4x4 xc:white -alpha transparent \
-channel alpha -fx "(1-min(1,(j+1)%2)) * 0.1" \
-write mpr:lines1 +delete -size 512x512 tile:mpr:lines1 \
\) -composite \
\( \
-size 4x4 xc:white -alpha transparent \
-channel alpha -fx "(1-min(1,j%4)) * 0.25" \
-write mpr:lines2 +delete -size 512x512 tile:mpr:lines2 \
\) -composite \
wallpaper.png
which yields this. You may see that pattern is broken (especially at the bottom).
If I set pattern size to 64x64 the problem is gone but that's not a real fix.
What am I doing wrong and how do I fix that without changing the pattern size?
UPD:
Full magick --version (convert --version) output:
Version: ImageMagick 7.0.8-23 Q16 x86_64 2019-01-04 https://imagemagick.org
Copyright: © 1999-2019 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC HDRI Modules OpenCL OpenMP
Delegates (built-in): bzlib cairo fontconfig freetype gslib heic jbig jng jp2 jpeg lcms lqr ltdl lzma openexr pangocairo png ps raw rsvg tiff webp wmf x xml zlib
Do the lines always appear at the same places from run to run
Yes, for each of the same set of arguments result is the same (but may vary for slightly different sets of arguments like changing resolution or removing write commands from braces).
Just as a test, try saving to TIFF rather than PNG. Does that works
or not?
Changing the output format always produces exactly the same result. I tried tiff, gif, jpeg. (With jpeg those lines are a bit blurred obviously, but result looks pretty much the same). This also applies to fmw42's command variant.
Do you have OpenMP installed with ImageMagick. Also disable OpenCL and see if that helps.
OpenMP is not installed but supported by the IM version I have.
Disabling OpenCL via environment variable MAGICK_OCL_DEVICE=OFF does not change the result. Recompiled IM with and without OpenCL and OpenMP from tag=7.0.8-23 and from master - all the same...
Are you seeing something in tmp4.png that is not visible to me?
Yes, I still can see that rainbow-colored line on white background if I do magick tmp4.png -alpha remove tmp4-opaque.png

Answer 1

Your command works fine for me on ImageMagick 7.0.8.23 Q16 Mac OSX with libpng 1.6.35. But in IM 7 use magick, not magick convert and not convert or you will get IM 6 behavior. Check your version of libpng and upgrade if necessary. What is your version and date of ImageMagick and on what platform?
magick \
\( \
-size 512x512 \
-define gradient:vector="0,0 0,720" gradient:"#5DA2C2-#C70AA0" +write tmp1.png \
\) \
\( \
-size 4x4 xc:white -alpha transparent +write tmp2.png \
-channel alpha -fx "(1-min(1,(j+1)%2)) * 0.1" +write tmp3.png \
-write mpr:lines1 +delete -size 512x512 tile:mpr:lines1 +write tmp4.png \
\) -composite \
\( \
-size 4x4 xc:white -alpha transparent \
-channel alpha -fx "(1-min(1,j%4)) * 0.25" \
-write mpr:lines2 +delete -size 512x512 tile:mpr:lines2 \
\) -composite \
wallpaper.png
Check this and see if it has your same issue. But I do not see any broken lines.