Discrepancy in rendering of content_subtitle block

I have a Django site I've been running for years (since before the 1.x days). Right now it's running 3.2 and I have "cloned" it to another Ubuntu 20.4 server so that I can test the upgrade to 4.x. The cloning process involved:

• pushing the site's code to a private GitHub repo
• git clone to pull the code down to the second server
• create a virtual Python environment
• in the new environment: pip install -r requirements.txt
• create the MySQL database and populate it from last night's backup

The version of Python is identical between the two servers (3.8.10, built Nov 26 2021, 20:14:08 with GCC 9.3.0 on linux). The version of Django is identical (3.2). The MySQL database server is at the same version (8.0.28-0ubuntu0.20.04.3) The site looks and behaves exactly the same, feature for feature, pixel for pixel, between the two instances with one puzzling exception. On the cloned site, an <h2>...</h2> subtitle appears between the <h1>...</h1> title (for example, "Change invoice") and the div containing the form's fields when editing an existing record for any record type. This subtitle does not appear on the form page for a new record, and (here's the puzzle) it does not appear on the form page for any record, new or existing, on the original site. I have tracked down the template which renders the subtitle to django/contrib/admin/templates/admin/base.html, line 89.

{% block content_subtitle %}{% if subtitle %}<h2>{{ subtitle }}</h2>{% endif %}{% endblock %}

What's the best way to go about figuring out why {% if subtitle %} would be true on the cloned site, but false on the original site? I'll feel a lot more like the testing of the upgrade is on a solid footing if the existing 3.2 instances start off behaving identically.

Thanks!

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