It seems that with cinnamon 3.2 they replaced the selectors .popup-menu-boxpointer and .popup-menu by the new .menu selector.
Have a look at the comments in original cinnamon.css file (line 113):
/ .popup-menu-boxpointer and .popup-menu are kept for compatibility
with cinnamon version under 3.2. Use .menu in version 3.2 and above /
If you define also a .menu selector in your css file the border issues should be fixed.
Although there are problems with the size of the popup-menu-arrow.
All properties of .popup-menu-arrow selector should be replaced by the property icon-size.
In my case this fixed the problem.
It seems that with cinnamon 3.2 they replaced the selectors .popup-menu-boxpointer and .popup-menu by the new .menu selector. Have a look at the comments in original cinnamon.css file (line 113):
If you define also a .menu selector in your css file the border issues should be fixed.
Although there are problems with the size of the popup-menu-arrow. All properties of .popup-menu-arrow selector should be replaced by the property icon-size. In my case this fixed the problem.