So I am guessing that yellow that he threw in was a small bounce from the 51?
possibly, from the 51, he dropped 100pts that morning, or maybe more? He might have dropped a bit earlier than normal and the 51 was him on his way up?
If you are at home at +2, I might suggest getting a test then if that number is lower than PS it's your signal that he is going to have an active cycle and you may need to test further.
Another reason for that yellow, might be the duration was a bit shortened, he was 168 at +8 so insulin cycle seemed to be petering out, then you gave him his meal, they can get a significant bump from food when given in the latter half of the cycle (as the insulin action is waning) that results in the the PS number popping up. This is something that we want to avoid since lantus works best when it can grab on to lower numbers, that's when you see well regulated kitties having those flat cycles that lantus is so famous for, take a look a Georges SS you can see what I mean by than if you look at 2016 and as he got regulated you can see how flat he stayed, earning reductions by staying in green for a week.
Watch out for those PS numbers popping up if you find it becomes a pattern, it might be worth changing the feeding times so that he doesn't get any significant food after +6. If like many kitties he gets a bit of acid tummy if he goes too long without eating you can try some freeze dried treats or some poached chicken breast, just a few small pieces to take of the edge, many find that a few pieces won't impact BG.