When the K6 is configured as an INPUT sensing GND or OPEN, and the VCC is required, the VCC must be referenced to the ground it is sensing. But is there a need to supply the reference ground to the K6 bank?

Yes, both VCC and GND are required to be supplied when using a pull-up (also GND is required for BIT).

Note: This also applies to the latest discrete I/O modules, DT1/DT4