I tried to do the same with the PFC8574 I/O Extender and got the same results. I made a post about it here: ESPHome + PCF8574 + Pulse Counter and quickly came to the realization that you cannot use pulse_counter through the I/O extenders. 
It would be really nice to be able to send everything ‘remote’ through I²C since I have found a way to send it very long distances with a set of P82B96Ps.