Hey Philip,
Thanks for reaching out to us!
This definitely sounds like a very strange issue to be having. I tested on my local install and was able to replicate the issue. It looks like this only happens when the main “events” slug and the slug for single events are set to the same string. In this case it looks like you’ve got yours set to both be “calendar” like this https://cloudup.com/ceX_7WY0UgD.
I’ve logged a bug ticket for this issue but in the meantime if you change the single to something else (maybe back to “event” or “calendar-event”) you’ll be able to use “past” in the event title.
I’m sorry for the inconvenience. Definitely a very strange and rare occurrence.
Thanks!