I lowered the capacitor to 47 pF. This is just enough, 100 pF would be better.
Then I did a full scan of this (in reverse)
The response is this:
A more detailed Excel graph is below.
This shows how the sensors react relative to each other. Important is that the trigger lies between the position encodings.
Also the trigger signal should not be covering to much distance.
SCAN CALCULATIONS
RowDistance : 12,52 mm
StartTriggerA : 415,02 ms
EndTriggerA : 425,84 ms
StartTriggerB : 477,78 ms
TimeBetweenTrigger : 62,76 ms
ScanSpeed : 199,5 mm·s⁻¹
ActiveTriggerTime : 10,82 ms
TriggerActiveDistance : 2,16 mm
Which is 2.2 mm, about the size of the trigger bar. This can be made smaller by using a darker reference.
This is important to know because the sensing of the magnets on the other side of the key must overlap this size. So that the data is always sensed when the trigger is fired.
What is also shown is that the sensors act about the same. The scanning was done while lifting the sensors a bit above the key. This influences the sensitivity of the sensor a lot, but using a reference sensor might mitigate the difference.
For this application the results are good enough. The circuit however draws 160 mA which is a bit much (battery power wise).