file type changed from python to json when starting editing

[chunxuan-hs]

Describe the bug
It looks nice when open the file with neovim. However, when starting edit the buffer, the file format changed from python to json. All syntax highlight lost. Edits are saved back to the notebook.

I set the format to python, but the problem happens with md as well.
Diagnostics

• vim --version: NVIM 0.9.4

• Operating system information (e.g. uname -a): Darwin MBP

• python --VV: unknown option --VV

• Are you using Anaconda?
  NO. miniconda

• Put let g:jupytext_print_debug_msgs = 1 in your ~/.vimrc. What is the output of :messages when reproducing the problem?
  NO output

• Does converting the notebook to/from ipynb with jupytext on the command line work?
  YES

• Does it work when you set g:jupytext_command in ~/.vimrc to be the exact some jupytext that you used manually, with the exact same version of Python?

• YES