Looks like a rendering bug was introduced.
http://javascript.daypilot.org/sandbox/scheduler/timeheaders.html
On last day of the month no right border is rendered.
It's not just the border, the background is also not applied in the last cell of the row.
I have the exact same problem, the entire last cell is never rendered.
It's now fixed in the latest sandbox build (7.9.1384):
http://javascript.daypilot.org/sandbox/scheduler/
An official SP release will be available early next week.