Hi Pilou - there isn't a real cap there, it is a display error due to the boolean creating what is called a "non-manifold" edge in that case.

A non-manifold edge is an edge that is common to 3 or more surfaces instead being common to only 2 surfaces.

The next beta has fixed this area of booleans so that they will not create non-manifold edges anymore - the results of booleans will be broken up into only manifold pieces.

In this case, the next beta will break your object back into its original pieces though - that's because you're trying to union a piece which does not enlarge the volume of the main piece.

Your lofted surface in this case would add no thickness to the main body of the solid - to union it into the existing volume of the solid at that shared edge location, you will need to give your surface some thickness (for example by using shell), and then union it. I have attached here the result of doing that.

- Michael