Makes sense. Though if user isn't precise they need to anticipate and deal with this delay often. Feels a bit insensitive and clunky as a new/returning user.

I tested it on Blender 2.79a and the behavior is the same.
I don't understand why it is said to work on it.
Do you use a pen, mouse, other...?

Using a mouse. On retesting 2.79a it has similar behavior but is less 'sticky' and drop-downs close easier when moving cursor away.
You can compare by moving the cursor straight up after a drop-down has been expanded. 2.79 will close consistently while 2.8/9 will stutter, sometimes closing, sometimes not.

This doesnt look like a bug to me.
Maybe this is different on Windows, but for me on Linux it will close those menus pretty consistently when moving the mouse straight down/up or left.
The only time it "hangs" is when mouse is approaching the menu (so mouse movement includes motion to the right).