Didn't find the description of the root-cause. The workable workaround happened to be push to remote branches in bundles, followed by git push --mirror:

git clone --mirror github.myorg.com/EXAMPLE-USER/REPOSITORY.git 
git remote add ssh_mirror github.myorg.com/EXAMPLE-USER/NEW-REPOSITORY.git git push ssh_mirror refs/heads/archive/2024/A* 
git push ssh_mirror refs/heads/archive/2024/B* 
.... 
git push ssh_mirror refs/heads/archive/2024/z* 
git push --mirror ssh_mirror