When on the date with Lexus and you get 7 Heart Stones to spend during the course of the evening. I just played through this twice in a row and it happened consistently. After adding up all the Heart Stones I should have spent it should have been all 7 of them, but somehow I end up retaining one and not getting full Lexus points and stat upgrades.
Pomper wine stuff = 3 stones
Replace dinner = 1 stone
Tip band for playing a song = 1 stone
Tip waitress = 2 stones
That all adds up to 7, so one of them isn't registering correctly.