Bottom line: cat /sys/class/power_supply/tps6598x-source-psy-0-003f/uevent
That will tell you what the actual result of the negotiation between the Librem 5 and the power source was. What protocol. What (maximum) voltage. What maximum current.
That will answer your implied question:
There is no problem with a power source offering voltages higher than 5V but offers can be rebuffed in the negotiation (and that should occur for any voltage above 5V).
The actual instantaneous charging current will change throughout the charging period (and hence likewise the instantaneous power). All other things being equal, when charging a relatively empty battery, it will start high (around 2A) and reduce (to maybe a few hundred mA by the time that the battery is almost full). So if you want to see maximum power, you need to start with a relatively empty battery.