I've tracked this down. It comes from an inconsistent template tag:

@register.filter(name="published_date")
def get_published_date(date):
    """less verbose combo of humanize filters"""
    if not date:
        return ""
    now = timezone.now()
    delta = relativedelta(now, date)
    if delta.years:
        return naturalday(date) # e.g. "Sept 14"
    if delta.days: # <= ONLY TRIGGERS IF WITHIN LAST YEAR AND NOT SAME DAY OF MONTH
        return naturalday(date, "M j") # e.g. "Sept 14"
    return naturaltime(date) # e.g. "an hour ago"

The intention of this tag appears to be that if the published date is more than 2 days ago, the actual date is shown, and if it's more recent it says "yesterday" or "4 hours ago" etc. The problem is that if the published date was within the last 12 months it won't have a "year", and if it was the same day of the month it won't have "days". e.g. today is 14 September 2024 by my calendar:

• published 13 February 2024: "Feb 13"
• published 14 February 2024: "6 months ago"
• published 14 February 2023: "Feb 14"

So this is broken both for dates older than a year ago (because it doesn't show a year) and for dates within the last year that are the same day of the month as today.

Hmm actually older than a year should be fine as it should default to DATE_FORMAT