I ran into the same flexbox conflict issue today. I noticed it when it would randomly stack the event list flex items at large screen views making featured images in the event list HUGE (full screen). Once I refreshed the page, the flex items would fall back side-by-side instead of stacked. A few times it would lock into the non-full-width thing the OP described.
I reverted back to version 2 for now. Least painful course for me at this time.
Yes, this may be a The Events Calendar issue, but you might want to keep this on your radar. Lots of folks using both this theme and The Event Calendar.
Cheers!