The following information comes from the latest Production Notes, translated by Perfectly Nintendo... - Tetsuya Takahashi (General Director) talks about an issue that popped up following the release of version 1.2.0 - after Chapter 7,
under certain
circumstances, Blades synchronised to a
specific Driver are not
displayed in the Blade List - no worries, the Blades have not disappeared - Takahashi explains that this issue occured due to the implementation of the «New Game +» mode, that will be added to the game in version 1.3.0 - that update will be released in mid-February - version 1.3.0 will fix the «disappearing Blade» issue, so you will have to live with this annoying but harmlesss glitch for now