On Mon, May 01, 2023 at 03:04:36PM +0800, Hongyi Zhao wrote:

[...]

> Another important clue, when the problem occurs, I noticed the
> following in the *Messages* buffer:
> 
>      font-lock-compile-keywords: Font-lock trying to use keywords
> before setting them up
> 
> On the other hand, the correct open TeX file didn't trigger the above message.

I don't even know what that message means: hopefully someone more
expert in Emacs things can tell us. On the other hand, I can't
imagine that this thing causes the file not being loaded.

What happens when you click on the complex TeX file and then list
the buffers in your Emacs (C-x b)? Does the buffer appear on the
list?

That would mean that the file is loaded, but Emacs still shows you
the *scratch* buffer. Otherwise, the file doesn't get loaded at
all.

This might give hints to find the problem.

Cheers
-- 
t