How did SE manage to read 9 upside down?
The following old bug report on Meta SE seems to be identical to what you observed: Activity log entry before registration date.
Though that report is marked status-completed, it seems that there is a regression of this bug on at least two other SE sites:
- Anime & Manga SE: According to the activity tab of my profile, I performed some actions on a site before joining
- Science Fiction & Fantasy SE: How could I create a tag wiki three years before I joined the site?
Looks like we can now add Mathematics SE to this list as well.