My MAX32620/21 RTC is running too fast and gaining 10 seconds a day. What should I look at?

Make sure you are using a CL=6pF crystal, instead of a CL=12pF crystal.  Make sure no additional CL capacitors are being used externally.  No external capacitors are needed on the 32KIN and 32KOUT pins.  The 32KIN and 32KOUT pins have an internal capacitance of 6pF each.  Therefore, only a 32.768kHz crystal (XTAL) should be connected to these pins.  The characteristics of the crystal should be CL=6pF and an maximum effective series resistance (ESR) of 70kohms.  If this does not address the problem, then the next step is to evaluate the PCB layout for the crystal.