A significant proportion of cheap brazed tooling isn't finish ground.
Sometimes they even have paint below the cutting edge, or you can see the cutting edges of the insert are at 90 degrees, which with an angles insert means they will rub.
This tool IS flat underneath, just it has been slightly rounded underneath at the front. You can see the cutting edge needs relieving.

Usually all that needs to be done is to grind some front relief, but for a beginner grinding carbide to a good finish can be a challenge.
Some of the cheaper sets are crudely forged and when the nominally correct size is used the cutting edge can end up way above or below centre height. My first set of these, each one required either a shim, or a mm or so milling off the bottom of the shank.
A better set I have has accurate cutting edge heights, but they still need a touch up on the cutting edges.
I'm perfectly aware you can buy precision finished brazed carbide tooling (its essential for big modern lathes) but they typically cost as much or more for one tool as for one of the budget sets.
My advice to all beginners is get a set of preground HSS tools to suit your lathe. At the most they may need a small shim to bring them to centre height (use strips of drink can).
These HSS tools are usually designed to be resharpened by grinding on one face only, which makes life easier for a beginner too.
When they come to grinding their own tools the pre-ground tools can be a guide to shapes and angles, although I would recommend using the 'knife tool' design for most general work as it is vastly easier to grind than the 'groove behind cutting edge' type included in the sets.
Neil