Replaced battery on Galaxy s8 still gets recognized as 3Ah
While the new battery is a higher capacity unit, the phone is not going to draw more power because of this.  It will still draw only what it needs (and clearly the old 3AH unit was adequate) not what the battery is able to source.
In most cases phones like this assume the battery is the one they are manufactured with and any information they display about the battery is based on assumption about its capacity.  So the phone was designed with a 3AH battery and still assumes that's there.
You should, however, see longer run time between recharges with the higher-capacity battery.  So if the old battery (when new) lasted 10 hours this new one should last over 3X that.