Did you see the warning when you unlocked the bootloader? It did say you voided your warranty. You could have relocked it before you sent it in the first time. When you unlock the bootloader and flash custom software it could do anything to your phone. The warranty is pretty much saying that HTC guarantees their hardware and software that runs on the hardware. But when you change what software is running on the hardware they can't guarantee that somebody else's software won't brick the device. HTC did you a favor by fixing the phone the first time. Its like if you bought a car and then put in your own engine in it. The manufacturer won't guarantee that engine because it isn't theirs.
OEM's don't make bootloaders so they can void the warranty. You void the warranty by your own actions
OEM's don't make bootloaders so they can void the warranty. You void the warranty by your own actions