This is a result of my changes in:

commit 85113fcda97970bc2468f409278e27d6570fc76f
Author: Mauro Aranda <maurooaranda@gmail.com>
Date:   Fri Jan 17 17:12:08 2025 -0300

     Prepare markers for insertions inside of a widget

     Recreating child widgets without recreating the parent widget
     may lead to situations where the parent widget doesn't cover its
     children or buttons entirely anymore.  This bug manifests as a
     faulty fontification of children or buttons, for example.
     (Bug#69941)

     * lisp/wid-edit.el (widget--prepare-markers-for-inside-insertion)
     (widget--prepare-markers-for-outside-insertion): New functions.
     (widget-default-create): Use them.

     * test/lisp/wid-edit-tests.el (widget-test-insertion-at-parent-markers)
     (widget-test-insertion-at-parent-markers-2): New tests.


But I don't think there's a bug in the wid-edit.el code, rather it's in
customize-themes.  The layout for a line with a theme is like this:
CHECKBOX BUTTON -- THEME-DESCRIPTION

BUTTON has the theme name as a label, and it disappears after toggling
the checkbox.  The code creates BUTTON as a child of CHECKBOX, so the
code is asking for BUTTON to be contained inside CHECKBOX.  It didn't
happen before because of Bug#69941, but I think that what happens now is
what should happen.  Now, when the checkbox gets recreated, it deletes
its child and never creates it back (the cus-theme.el code should be
responsible of telling it to create it back in that case).

But I don't think BUTTON should be a child of CHECKBOX.  In fact, the
usual thing to do with the Widget library is to make it a sibling.  So I
propose the attached patch.