In the buffer loaded as above, with the wrong fontification, I have:

major-mode is a variable defined in ‘C source code’.


Its value is ‘latex-mode’
Original value was ‘fundamental-mode’
Local in buffer test.tex; global value is fundamental-mode
Then I suggest to look for the reason in json-mode.el.  For example,
this part:

  (add-to-list 'magic-fallback-mode-alist '("^[{[]$" . json-mode))

could change the global state when you load json-mode.

I opened and issue on the json-mode GitHub project page.