Setting a custom cursorconceal value in an after/ftplugin does not "stick" when a buffer is loaded via bufadd + bufload. Other values from the ftplugin are set correctly.
In the example configuration, there are two important settings,
concealcursor
The init.vim value is niv, meaning all concealed text should remain concealed when the cursor hovers it.
The after/ftplugin/markdown.vim value is `` (blank), meaning concealed text should become visible when your cursor moves to the line containing concealed text.
showbreak
The init.vim value is _, meaning wrapped lines should be prefixed with _.
The after/ftplugin/markdown.vim value is *, meaning wrapped lines should be prefixed with *.
You must remove any existing /after configuration files, and add the repos included /after files, when running neovim.
nvim --cmd "set rtp-=~/.config/nvim/after" --cmd "set rtp+=~/concealcursor-bug.vim/after" -u init.vim
Now you may press "b" in normal mode. The file sample.md will be loaded via bufadd() and bufload() and nvim_win_set_buf().
If you navigate the file, to the indicated line, you will see no concealed characters. Wrapped lines will however, be correctly prefixed with _.
If you inspect :verbose set showbreak?, you will see it was set via the ftplugin file. If you inspect :verbose set cursorconceal? you will see it was last set by the init file. If you inspect :set ft?, you will see it is correctly detected as markdown.
Quit vim.
Reopen using the previous command.
Now you may press "n" in normal mode. The file sample.md will be loaded via the :edit command.`
If you navigate the file, to the indicated line, you will have concealed characters revealed to you. Wrapped lines will also be correctly prefixed with _.
If you inspect :verbose set showbreak?, you will see it was set via the ftplugin file. If you inspect :verbose set cursorconceal? you will see it was also set via the ftplugin file.
If you press "b" to view the bugged load, then run set ft=markdown, the correct concealcursor will be set.