The FORCE2 pin is not used in 2 and 4 wire modes, and does not connect to the internal ADC (indicated by a dotted line in the Block Diagram on page 8 of the datasheet). However, we do not want to let an input float (ie, be unconnected). If left unconnected (floating) there is a chance that the voltage on an unconnected FORCE2 pin could exceed the high voltage threshold (ie, > VCC) and cause the MAX31865 to think there is a high voltage issue, and open the protection gates to the RTD pins (the over/under voltage protection circuit). To prevent the floating condition, we state that FORCE2 should be connected to GND if not used when in 2 and 4 wire configurations.
However, in the MAX31865 evaluation kit, we have FORCE+ connected directly to FORCE2. This is required for 3 wire configurations. We set bit D4 = 1 in the configuration register to enable the ADC to read the voltage on FORCE2 to determine the IR loss on that leg of the RTD cable. When D4 is set to 0 (for 2 and 4 wire modes), FORCE2 is no longer connected to the internal ADC, so we don't take a measurement of the voltage on the FORCE2 pin. So if we have FORCE2 connected to FORCE+, that is ok since we aren't converting the voltage on FORCE2 or using it to adjust for IR cable drops in 2 and 4 wire modes.
While we recommend FORCE2 be connected to GND in 2 and 4 wire modes, it is actually ok to have a voltage on FORCE2 that does not exceed VCC or GND. We just want a stable potential on FORCE2 so it doesn't float. So applying the FORCE+ voltage to FORCE2 in 2 and 4 wire modes is fine since we are not monitoring the voltage on FORCE2 (D4 bit = 0). We just do not want FORCE2 to be left unconnected.