ANdrei,
In your video, you did not trim the top surface "all the way back"... You started at the back seam edge.

When he trimmed it all the way back, he was having issues.

The 2 surfaces at the back (converging area) are not really tangent, or equal, so the blend back there is producing the poor results.

He fixed it by trimming out a larger portion of the top, AND bottom. That is kindof "hiding" the stressed surface.

The best thing would be to re-create that object, and have the back "converged" area come from "one initial piece". Either the bottom piece, or the top piece. One or the other.