Stack Logic Problem....

I'm making an image editor in Android Studio Java. I am making an undo and redo features by having an undo stack - which every change in the image is added onto, and a redo stack - every undo, the image is stored there. The problem I have is for some reason, for any amount of undos I do, the last redo will never work. It doesn't update the image.

For example: I change the image from black to white, then undo. If I try to redo, the image will stay white.
But, if I do black -> red -> white, then undo twice (back to black), and redo twice, it will change to red but not change to black after.

This is my code:

    public void undo(View view) {
        if (!undoStack.isEmpty()) {
            // Save the current state before popping
            redoStack.push(bitmap);

            // Restore the last saved state
            bitmap = undoStack.pop();
            img.setImageBitmap(bitmap);
        }
    }

    public void redo(View view) {
        if (!redoStack.isEmpty()) {
            // Save the current state before popping
            undoStack.push(bitmap);

            // Restore the last undone state
            bitmap = redoStack.pop();
            img.setImageBitmap(bitmap);
        }
    }