0# is part of indentkeys in the default yaml.vim (some /usr/share/vim/…/indent/yaml.vim). On opening a yaml file, this will overwrite the indentkeys setting in your ~/.vimrc.
My first idea is to include an autocmd in your ~/.vimrc on opening yaml files:
autocmd bufread *.yaml set indentkeys=!^F,o,O,0},0],<:>BufNewFile,0- or just
autocmd bufreadBufRead *.yaml set indentkeys-=0#