At such a low Vin, it's not likely we are going to hit the peak current limit, but we're most likely hitting the max duty cycle limit .
Let's say the Vin is 0.4V , and Vout is 3V , if we assume 90% efficiency , the part will try to go into continuous conduction mode (CCM) for load currents greater than 1.9 mA .
If you calculate the CCM duty cycle for Vin=0.4V and Vout=3V, it is 88% which violates the 75% limit ( given in the EC table) , which means Vout will start dropping out of regulation .
Regarding operating the MAX17222 with 0.4V input, there are a few things to keep in mind :
1. During startup the Vin has to be greater than 0.88V as mentioned in the EC table.
2. After the part has started up, the Vin may drop to 0.4V (typ), and the MAX17222 will still regulate the output voltage as long as the load is very light (<1mA)
3. Also, the EN should not be tied directly to Vin when Vin drops to 0.4V , because the part will get disabled when EN goes below 0.5V .