@dc42 Thanks for your response.
The board is running Klipper, not RRF, but I know that the setup & configuration is just fine. Note that I am using IO_2 pins 2, 3, 4 & 5 for BL Touch interfacing.
The issue itself is multi-fold and very random in nature: once every few days Klipper randomly fails to establish comms with BL Touch posting "failed to verify sensor state". Several times it also failed to recognize the pin near state during homing, which resulted in a hot end crash (good thing I home with very reduced stepper current). Between these events the printer works fine for days with no issues - including multiple multi-probe 10x10 grid bed geometry measurements with all heaters and steppers on.
It all seems quite random in nature and my prime suspect is EMI & electrical noise due to coupling. I was really shocked to see 0.85V p-p noise on the BL Touch 5V power and SIG lines when the steppers are powered-up, even when not moving. I separated heater & stepper wiring from the signal wiring to reduce any potential coupling. This dropped the noise magnitude to 190 mV p-p. I also switched the BL Touch (v3.1) to 5V mode to improve the noise margin. I don't want to go totally nuts and start using shielded BL touch wiring, so the cap in proximity to BL Touch is meant to be additional "goodness" so that I can start trusting the printer again...
Thank you.