Hey mutinii, thanks for the support :)
That is actually intended; in those frames, the huge sword is touching the ground, so the black outline disappears when the edge and the ground touch.
However, I understand you might prefer the sword to keep the outline during the whole animation. Fortunately, you can easily edit the sprite sheet to add the black pixels for closing the outline in Aseprite, Pixel Edit, Photoshop, or similar software.