Problem after upgrading to 13.1.3

[Greg Bognar]

Hi,

I've just upgraded to 13.1.3 from ELPA.  I'm on Emacs 27.1.

When I open a LaTeX file, it opens in something called Fundamental mode.  But I
do actually get font-locking and the basic AUCTeX menus, but none of my
customizations seem to have loaded.  C-c C-c asks for the master file, even
though TeX-master is set in the file-local variables.  M-x describe-mode tells
me:

   Fundamental mode defined in `tex-mode.el':
   This function has :override advice: `TeX-latex-mode'.

I use use-package to load AUCTeX:

   (use-package tex
     :ensure auctex
     :defer t
     :mode ("\\.tex\\'" . LaTeX-mode)
     [...]

When I try to change the mode of the file by M-x LaTeX-mode, I get an error:

   Cannot open load file: No such file or directory, tex-buf

My setup has worked without any problems before the upgrade.  It seems
use-package does not load AUCTeX properly, although this is the way recommended
in the manual.  I should also say I have a cryptic comment in my init file:

     :ensure auctex ;FIXME: cannot be installed automatically

But I can't remember what problem I was trying to fix.

Does anyone who uses use-package have any advice on how to get AUCTeX working
again?

All the best,
Greg