Phone displays have gotten taller in the past few years, and it seems the OnePlus 6T might not make use of the entire screen by default for a few games. Most games play nicely with the large display on the 6T and its teardrop camera notch, but Pokémon GO is one that doesn't scale correctly. Until an official fix happens, there's a quick workaround you can use to make things a bit better.
The augmented reality feature in Pokémon GO is one of the key components that draw people in to the popular Android and iPhone game. Indeed, there's nothing quite like the sight of a Pokémon standing around in an otherwise uninteresting parking lot.