FWIW even though it’s rated for 4.35V, the battery should only be charged up to 4.2V because the modem (and perhaps GPS module too? Would have to check to be sure) is only rated up to 4.2V.
(unless you don’t use such battery with the Librem 5, then feel free to charge it up to 4.35V
)
Other than that, it’s normal and expected for slower charge to end up with higher capacity. You can slow Librem 5’s charging down too by manipulating /sys/class/power_supply/bq25890-charger-0/constant_charge_current.