reproduce step

minivimrc

set nocompatible
set conceallevel=2
syntax on

1. `vim -u minivimrc minivimrc
2. :normal! gg
3. call matchaddpos('Conceal', [[2, 19,1]], 10, -1, {'conceal': 'f'})

you will see:

I can not understand why the content of first line will be shown after conceal cchar?