operator-surround-replace kills indent in visual mode

[Konfekt]

Let .vimrc contain

nnoremap <script> <SID>(hvgu) ^vg_
nmap <silent> css <SID>(hvgu)<Plug>(operator-surround-replace)

and let test.vim consist of the single line

  (nmap csg)

Expected: hitting cssB gives

  {nmap csg}

Result:

{nmap csg}