It turns out that SendGrid—since it is my email backend (django-sendgrid-v5) and was being used when mail_admins() was being called by the Django's logging module (django.utils.log) every time a server error occurred—was taking anything that looked like a URL and replacing it with a click-tracking URL because django-sendgrid-v5's default behavior is to tell the SendGrid API to do that.

The solution was to set these variables in settings.py:

• SENDGRID_TRACK_CLICKS_HTML = False
• SENDGRID_TRACK_CLICKS_PLAIN = False