Different fontification in temp buffer

[Daniele Nicolodi]

Hello,

I'm hacking on a minor mode and I'm writing unit tests for the
fontification feature. I am encountering a strange issue: the
fontification is different if execute in a temp buffer or in a regular
buffer. In particular:

  (with-temp-buffer
    (insert string)
    (fundamental-mode)
    (beancount-mode)
    (font-lock-ensure)
    (buffer-string))

and

  (with-current-buffer (generate-new-buffer "*test*")
    (insert string)
    (fundamental-mode)
    (beancount-mode)
    (font-lock-ensure)
    (buffer-string))

strangely result in two different results. Why is it so?

Thank you.

Cheers,
Dan