Upgrade breaks Rest Export View in Calendar
My edit gave me a clue to look at the date formatting. In our specific calendar instance there is an event-calendar.js file that was manually converting the dates pre-Drupal 8.7 to UTC with a .field_date_range[0].value+'+00:00',. It appears that Drupal 8.7 does this automatically, so removing that extra bit of code solved the issue and dates are being populated into the calendar correctly now. 

EDIT: For those interested, I was finally able to find the drupal documentation about this change in 8.7 here