Incorrect calendar on external monitor

I create a repeating calendar event and then I choose to end it on a given date. This brings up a mini calendar from where I can choose the ending date. If the Calendar window is on my main display (MacBook Pro) then the mini-calendar is correct. If the Calendar window is on my external display (Dell) then it is incorrect. I attach two screenshots.

This has to be one of the most bizarre bugs I've ever seen on a Mac.