My older opus will take battery’s to 4.20+ think it a 2.0 or 2.1 Version ,but this newer version (2.2) will only charge to 4.18 ?
Double check with voltmeter batteries definitely undercharged .
4.18 is within specs for charging lion batteries.
Gee, maybe your cellphone battery is only charging up to 4.18V 
But you have no way of knowing that… So it doesn’t bother you. 
BTW, there is a way to raise the final charging voltage by 0.05V. I’ll leave it to you to search that out. It involves opening up the unit and soldering a bridge that put there for just that purpose. I believe the OPUS was “detuned” by 0.05V to accommodate for the plus and minus errors in production.
If you load the cells full to 4.2V and switch the light on, it takes <1s to drop the voltage down to 4.18V.
You don’t loose any capacity.
The charger is charging fine.
Opening the charger and soldering ,not this guy,lol
The Opus BT C3100 V2.2 is currently on sale for the next 5 days from this site for a grand total of $33.14 (US Plug) with free shipping.