Sometimes command outputs to preview window, other times echo'ed

- [x] I have fzf 0.23.0 or above
- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [x] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [x] I have read through the manual page of fzf (`man fzf`)
- [x] I have searched through the existing issues

## Detail
Sorry about the ambiguous title, its a hard issue to describe.

Basically if I execute `:Git stash list` the stash list is echoed out below the existing buffer however If i select the same command from the command history of `:History:`, the output is printed in the default vim preview window.

If i search previous commands with `:History:`, then instead of executing the command with an `ENTER` I hit `Ctrl-e` to edit first, in command mode, the output is echoed out as expected. Screenshots attached.

### Expected Behaviour

Occurs:
- When executing the command directly
- When finding command via `:History:`, hiting `Ctrl-e` and finishing with `ENTER`
- before latest upgrade

### Unexpected Behaviour
Step 1
Step 2

Occurs:
- When finding command via `:History:` and hitting `ENTER` directly

### Versions
- Fedora 33
- `vim-enhanced-2:8.2.2875-1.fc33.x86_64`
- `fzf-0.27.2-1.fc33.x86_64`
- fzf.vim is at `e34f6c1` from Tue May 25 13:30:41 2021 +0900

Here is a paired back vimrc that replicates the issue (on my system)
```
call plug#begin('~/.vim/plugged')
Plug 'tpope/vim-fugitive'
Plug 'junegunn/fzf', { 'do': './install --all'}
Plug 'junegunn/fzf.vim', { 'depends': 'fzf' }
call plug#end()
```

Any hints on this would be much appreciated, as is fzf and fzf.vim by the way. Thanks for sharing your work.