I tracked this down to a bug in emacs, which happens only when indirect buffers are cloned. Reported as bug#53294 with a patch. 

On Thu, Jan 13, 2022 at 9:51 PM Andrew Hyatt <ahyatt@gmail.com> wrote:

Hi all,

I've been having an odd problem where if I try to change my text
scale on my org capture buffers, the scale keeps increasing, and
increases the scale on the parent of the indirect org-capture
buffer, on each capture.

I'm using org-mode 9.5, emacs 29.0.50.

Debugging into the issue, I realized I can only reproduce this if
the org-mode buffer has variable-pitch-mode on.

Here's a short repro that works on emacs -Q (of course, you may
want to change the path):

(require 'face-remap) (defun ash/big-font ()
  "Creates a font that is big enough for about 20 lines of text."
  (interactive) (let ((text-scale-mode-amount (/ (frame-height)
  20)))
    (text-scale-mode 1)))

(require 'org-capture) (add-hook 'org-capture-mode-hook
#'ash/big-font) (setq org-capture-templates '(("c" "Capture" entry
(file "~/Desktop/test.org") nil))) (find-file
"~/Desktop/test.org") (variable-pitch-mode 1)

After evaluating this, every time you call org-capture, the text
size will increase, until the text size is so large emacs crashes!

I have also tried to reproduce this without org-mode, just using
indirect buffers, but can't do it.  Something that org-mode is
doing is causing this, but org-capture is complicated enough that
I can't really see what it could be.  I can and will continue
looking into this, but I'm curious if anyone might have a clue as
to what is going wrong here.