When g:NERDAltDelims_java is set to 1,sometimes it will use another comment style

[termanary]

nerdcommenter:Jul 31 2018 -> Date from git.
It is the version I am using now. And I download the newest one, the problem still exist.
vimrc:

filetype plugin indent on
let g:NERDAltDelims_c = 1
let g:NERDAltDelims_java = 1

shell cmd: $ vim vim.vim or $ vim py.py

I think the filetype I am opening is unimportant.

vim cmd: :vs java.java or :vs c.c

The problem will show up, too.
type:<C-w>l<C-w>h
Jump to the window of vim.vim then jump back.

Then nerdcommenter will use another comment style.
I have seen some similar problems in the issue list.#342 is a similar problem.
I found that the b:NERDCommenterDelims is been modified unproperly.
The reason is the function s:SwitchToAlternativeDelimiters is called.
But I then could not find out where and why it is call.

[alerque]

This is probably a bug in relation to variable scope. We're probably doing something with global variables that we should be keeping to buffer scope. I'd love to see this rooted out if anybody wants to contribute a fix for that.