iPhone 8 not charging properly

Your iPhone 7 charger should be as good as one supplied for iPhone 8 - in fact they are probably the same accessory for both phones. Reading your post I suspect a software issue which could be either early stop on the charging, or it could be the % indicator has got confused. There are many other posts about this problem if you use our search tool, Apple - Support - Search so maybe some more ideas there too.

For a software problem the first thing to try is always a restart of the phone. Pick the method appropriate for your phone, If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support and let the phone reboot. You could also wipe the phone back to factory configuration and setup again, BUT DO A BACKUP FIRST! In fact you want a backup before you go back to the service center because they may wipe your phone without even asking about backup.

What happens if your also use the USB port of your computer and see, if this might rule out a problem with your adapters or USB cable?

But it is strange that they did not check your adapter. Maybe visiting another Apple Authorized Service Provider might also change the way they check your device and accessory.

You should visit them again, this time asking them to test your phone, using your accessory. And tell them about your findings when you tested your setup at home in every possible combination, Apple adapter+USB cable, USB cable+USB port computer.

Yes, sometimes recalibrating the battery by letting the phone shut down, using up the battery life, might help.

But this should not be necessary, if your phone is brand-new.

Did you try a different Apple Service Provider, in case they are more eager to find the issue?

This is not how charging a battery should have to be done.

I would try this once again, and, if the issue comes back again, take it to the Apple Service Provider and tell them what you tried to get the battery loading completely.

Good news 🙂