IMO don't trust those handphone retail shops
Best is go to the original stores such as SingTel, Starhub, etc
This is because you would not know how or where those retail shops get the phones. They could be illegally smuggled in or could even be "custom made"
My cousin had a similar experience where he got a phone below RRP and after he opened the casing, he found that the interior parts are not original which mean that only the cover was original. Even the battery was those "use a few times only" type.
We went to the service provider to check out the creditability of the phone and sure enough, they denied any originality in the parts.
As for your case bro, ask around for people who had similar incidents but have not reported to CASE yet and file a lawsuit against the retailer together