When I try to add a mating, SW cut my piece, as I show you in the video attached.
I'm not sure, if you have external references on this part try to lock them.
And how can I know if I have external references?
If the part have features that are done in context on another part or assembly. RMB on part -> List External Refs....
To be honest, I don't know if I understand exactly what a external reference is. But I have show the external reference, I have broken then all and I have solved the problem.
So, thank you for focusing me to the problem solution.
If this helps to understand the problem. I see the hole croquis is now out of the piece.
Why? I don't know
(Unless you changed the default colors) the fact that the sketch is blue means that it is under-defined. You need to add dimensions so that those features are fixed with respect to the part.
Retrieving data ...