@Gordon Checked with a multimeter, connections all look good, no bridges. Voltage is at ~3.3v on the 2 i2c pins, all the address pins on the PCA9685 are pulled down to ground with a 10k resistor and I'm using address 0x40. I switched out the 10k pullup resistors for 2.7k pullups with no change.
Doing a test with a digital write on each pin - when setting both on/off, both go 3.3v/0v. If I toggle one & not the other, sometimes they bind together, but when they don't I see minor fluctuations on the pin not being set when the other one changes, I'm guessing that'll be the bleed fanoush was talking about.
Is there anything else it makes sense for me to try, or is it time for me to bodge-wire to another 2 pins for i2c on the MDBT42Q? :)