Not sure if it is related, but when I created an ellipse, then a square and used boolean add to these, then alt-drag it to the right and add a rectangle in between to make a handlebar, i found out that when converting the handlebar to curves, the corners behave odd, and bug out the object.
Selecting both corners in the corner tool and dragging it has the following effect:
Note that when I undo this action and only do that on the right corner it works as expected:
However, when moving on to the second corner, it instantly reverts back to the inverted state (without touching it):
When alt-dragging the whole bar to create a duplicate, it looks like there is 1 point (not selectable by the corner tool) that is stuck
Now I'm a beginner so I may just be doing something horribly wrong and not have noticed.
If not, I believe this demonstration should be useful to debug the problem regarding boolean operations preceeding corner operations.
Hope this helps.