You need to know how much current the converter was putting out. Even in boost mode, it can be current limited to about 55A or so. (I've read that sometimes the PD converters are a little bit optimistic in their current ratings, so it could be current or power limited at somewhat under 55A....but it's the same general idea.) If this is happening, your converter is working as designed, and as the battery charges the voltage will rise up to the point where the output is being limited by voltage, rather than by current.
Please note that there's a difference between current (Amps) and charge (Amp-hours). Current is an instantaneous reading, like voltage, and the battery monitor is measuring continually. You can see it at any point by cycling to the "AMPS" display value.
