Was there an OS update that required users to reboot? Chrome doesn't tend to update until restarted - if people keep it open on a laptop (like I do) it can become outdated.
If there was a macOS or Windows update that forced this close, a bulk chrome update might have happened?
Except 69 was released over a year ago (2018-09-04), so that would mean that many people kept their OS without rebooting for longer than a year.
It is more likely than some unknown bot was using the old chrome identifier, and they just updated it.
Given the difference in versions is over a year old, I believe it was just an update to the User-Agent header that some unknown web crawler did on that day. You could identify it by comparing traffic before and after, and look if they have common source IPs.
Your traffic patterns look really different than what I see in my graphs; regularly within ~3-4 weeks of a major Chrome release, about 80% mobile and 90% desktop users already have it (JS-based tracking; this is the case today for M81, whereas at yours it seems to be at ~20%🤔)
I would look for something that check how unique are those requests. E.g geo IP information, subnet, ISP ... something that can say if it's a crawler. It's really interesting that the rest of the versions distributions didn't change dramatically.