The fun fact (not really) is that it’s still not fixed :(

As a fast workaround I can advise only to press Unite key in the Pathfinder panel. Even if its Remove Redundant Points option is turned off, this effectively merges these doubled points together.

This solution obviously does not work for several objects selected. If you have a lot of these, maybe using very handy VectorFirstAid plugin from AstuteGraphics pack can be beneficial (it does much more that this).

Well, 4 more years, I guess :(

You Sir deserve an award. That is exactly what happened with a gear I created. I went to round every corner around the external edge and one corner would not round with every other corner. I ended up zooming in excessively and found that I had two anchor points almost on top of each other had to pass 8000% zoom to find that. deleted the suspect one which did not open a path to me(lucky guess).

This corner was created by layering a rounded rectangle behind a circles edge then using the shape builder tool to combine the overlapped section of the circle to the rounded rectangle and then deleted the rounded rectangle giving me one valley between teeth.

Does the fact the corner of the complex object being created by using the shape builder tool contribute to it?

I am having the opposite issue. I want to eliminate the live corner radius numbers and have the object simply be a shape without a corner radius assigned (Just expanded shape to a path with a fill) so that if its scaled by just about anyone it can't get screwed up. It seems like CC limited the ability to do this easily, its making me insane! Just like you would outline a stroke, I want to "outline" or expand the corner radius created with the widget, any ideas?

Can I get a reply from Adobe staff please. This is a major bug and I have already told you what needs doing to fix it. What is happening, is someone working on this or is it just going to be ignored?

Update:

I've found the Root cause of the Bug which causes missing corner widgets on expanded shapes - so should be easy for your engineers/coders to fix now I've done half their job for them...

Problem:

When Illustrator expands a shape with an inside or outside stroke (not a centred stroke) it creates an extra, redundant anchor point in one or more corners.

For instance, when a rectangle with an inside or outside stroke is expanded, the resulting compound shape has 2 paths formed from the stroke (an inner and outer rectangle).
The top left & bottom left & bottom right corners of both paths have 1 anchor point each and a corner widget, which is exactly what should happen.
However the top right corners of both the inner and outer paths have 2 anchor points each instead of 1. In each corner the extra anchor point is in exactly the same position as the correct anchor point - ie. one anchor point lies directly on top of the other anchor point - so that the top right corners of both the inner and outer paths look like normal corners but in fact technically they are not corners since the path joins 2 anchor points in the same position. This is why there are no corner widgets.

Workaround:

Delete/Remove the extra anchor point from the corner (this may create an open path, if so then close the path)

The corner is now repaired and the missing corner widgets appear.