You mean when DAC output falls below 0.7V without VFD load, it is pulled up to 2V by the VFD's analog input? That is a bit unusual and could suggest some problem with the VFD's analog circuit?
If you add a 10K ohm resistor between the DAC analog output and the VFD analog input, you will then be able to measure the voltage at the F2424's analog output and the VFD's analog input and see if there is a difference. You can also calculate how much current the analog input on the VFD draws from the DAC output on the PLC at various SETDAC values.