thought this would be a better alternative to the pricier Samsung Galaxy phones, but just recently a small dark spot or shadow appeared in the top left corner of my phone

The dark spot/shadow doesn't show up on printed pics or screenshots, it appears on the screen only.

After just a couple of weeks of use, however, it started to show a cloudy, purplish spot on the screen, which continued to grow and grow and now occupies more than half the screen

Then it got an ink leak (a purple spot) and now its spread all over the screen

So even though this phone is more than capable of utilizing VoLTE, AT&T limits the functionality