22.2.3 Software Interrupt Mode
When the Capture mode is changed, a false capture interrupt may be generated. The user should keep the CCPxIE Interrupt Priority bit of the PIEx register clear to avoid false interrupts. Additionally, the user should clear the CCPxIF interrupt flag bit of the PIRx register following any change in Operating mode.
Important: Clocking Timer1 from the system clock
(FOSC) should not be used in Capture mode. In order for Capture mode to
recognize the trigger event on the CCPx pin, Timer1 must be clocked from the instruction
clock (FOSC/4) or from an external clock source.