Copper foil splits often occur at tight inside curves. This is caused by the inability of the copper to stretch enough at the edges to make it around in tact.
There are at least two ways to deal with copper foil splits: avoid them or repair them.
One way to avoid splits at tight inside curves is to hand foil pieces that have these curves. When folding the foil from the edge to the sides of the glass, gently and progressively ease the foil toward the sides of the glass with your fingers rather than a hard tool. Do not try to make the fold all at once. Give the foil the chance to stretch into the curve through gentle persuasion.
Another way to avoid the splits is to use several short strips of foil at the tight inside curve. Apply each strip from one side around the edge to the other side of the glass. Overlap each strip until the inside curve is covered. Then you can cover the whole curve in the normal way with the copper foil. When the piece is foiled, you can then take a craft knife and trim the overlapped foils to match the edge of the long strip of foil. Or you can just trim the overlapped foil pieces to match the rest of the foil. Be aware that this increases the space required between the glass pieces as the foil is now effectively twice the thickness of regularly applied foil.
To repair splits in the foil at tight inside curves that have been foiled along with the rest of the piece, you take small pieces of foil and overlap at the splits.
When tinning these repairs, you need to take care that the heat from the iron does not loosen the adhesive so much that the foil pieces move.