On 9.3.0 I do observe the reported behavior, and I wouldn't expect it either. Granted, expecting a right-click on something that indicates it has a sub-menu to do anything at all had not occurred to me until I saw this thread.
The only thing it would have occurred to me to guess it might do if you'd just told me it did something would be pin that specific submenu open so that it didn't disappear after making a selection, and that would have been a pleasant but (for me) not very useful surprise.
Then again, a year or two ago I wouldn't have expected left-clicking to do anything either, unless hovering didn't open the sub-menu, in which case I'd have expected left-clicking to do that.