Early signs of hardware failure...without being more specific, ffs.
At the end of the day there are idiots everywhere, you clearly bumped into one at your service centre.
BUT, you don't know if your device has a fault or not, sure staying on iOS 11.3 might have not made you discover it or it could have.
Guess its one of those things that just hard do prove.
Such as when a relatives iPad air 2 only charges to 70% after I updated it to iOS11.
Sure, it can be a bad battery and it might have been iOS 10 not displaying the charging process properly, still quite annoying.
Although I know its frustrating, talk to Apple support again and see what they can offer.
The times I have had a humble approach to them they usually have helped me or made a good deal on a new product.´
Good luck.