I got in contact with HTC through their website (did a live chat). They were very polite and asked for the IMEI number, a copy of the purchase reciept and some pictures of the phone. This is when I noticed some dark kind of leaking stuff through part of the transparent back cover. They advised me not to use the phone anymore because it might be dangerous. The warranty was not valid anymore as I bought the phone online and it wasn't a locally manufactured unit (I'm in Australia and apparently the phone was shipped from Hong Kong), was outside the 12 month warranty limit, etc, etc. They wanted to charge me an upfront $65 USD Global Warranty Service Fee (for shipping of parts for overseas variants) and a $49.50 AUD Diagnostic Fee (Assessment Fee and Freight charges back and forth) excluding the possible additional quotation for the parts that would need replacement (which would have cost who knows how much).
Needless to say I've decided to ditch the HTC U12+ and I've gone out and bought a Samsung S10+, which I'm still getting used to but like a lot. Luckily I copied all my data from the U12+ last weekend, because just tonight I've noticed it's suffered a catastrophic failure. What I assume is the battery has expanded and delaminated the back cover completely off the phone in the top corner, across the top edge and down the side opposite the one with the buttons, and it doesn't turn on anymore. I watched the screen flicker and die.
I still have no idea what caused this failure, but it definitely appears to be battery related. I've been using mobile phones since 1998 (exclusively HTC smartphones since 2010) and this is the first phone I've had destroy itself on me for unknown reasons. For a phone only about 15 months old it's pretty dissapointing.
Anyway, the S10+ seems like a pretty good one - I only hope it lasts longer than the U12+ did!