Mov to MP4 produces weird artifacts if mov has an alpha channel (transparency)

[Nommalorel]

Summary of the bug:
When i try to convert my animation .mov file that i rendered from spine2d that has a transparent background to an .mp4 file ffmpeg creates weird artifacts on and around the character.

First frame that is converted by adobe media encoder: ​https://i.ibb.co/mTyZ3W5/adb-md-enc-f1.png

First frame that is converted by ffmpeg (or any FOSS converter that uses ffmpeg as a base) : ​https://i.ibb.co/hR4PmVz/ffmpeg-f1.png

How to reproduce:

Render a mov file from any animation or video editing program with a video that has an object or a character at the middle but has a transparent background. Then convert with the code stated below.
% ffmpeg -i {inname}.mov {outname}.mp4
ffmpeg version
ffmpeg version 7.0.1-full_build-www.gyan.dev Copyright (c) 2000-2024 the FFmpeg developers
built with gcc 13.2.0 (Rev5, Built by MSYS2 project)
configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enable-libdavs2 --enable-libuavs3d --enable-libxevd --enable-libzvbi --enable-librav1e --enable-libsvtav1 --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxeve --enable-libxvid --enable-libaom --enable-libjxl --enable-libopenjpeg --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-dxva2 --enable-d3d11va --enable-d3d12va --enable-ffnvcodec --enable-libvpl --enable-nvdec --enable-nvenc --enable-vaapi --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libcodec2 --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint
libavutil      59.  8.100 / 59.  8.100
libavcodec     61.  3.100 / 61.  3.100
libavformat    61.  1.100 / 61.  1.100
libavdevice    61.  1.100 / 61.  1.100
libavfilter    10.  1.100 / 10.  1.100
libswscale      8.  1.100 /  8.  1.100
libswresample   5.  1.100 /  5.  1.100
libpostproc    58.  1.100 / 58.  1.100

built on ...
Windows 10