Optical encoder triggers false interrupts
The schmitt trigger you are specifically using hasn't got that great a differential between trigger threholds. If you look at the data sheet you will see that for 4.5 volts operation Vhi has a min value of 2.16 volts and Vlo could be as high as 1.79 volts. 

That's a range of 370 mV and quite likely is too low. However, the DS does indicate that the likely worst case differential is 0.71 volts but it's food for thought.

Standard CMOS is probably better but, of course, you will struggle to find that tiny 1-gate part.