Make sure you adjust the center bolt on top first while the 2 stem bolts on the side are still loose. Only tighten it a little, until there is no more headset play. That preloads the bearings, and then the 2 stem bolts lock everything in place. Do not tighten the stem bolts first. That will make the center bolt unable to preload the bearings.
See this video:
Altough, I'm wondering if your stem has the same diameter as the fork it's clamped on. If the stem has a larger diameter, it is not compatible with the fork, and would slide around no matter how much you tighten it, making the handlebars turn without the wheel.
There are these standards for threadless stems:
3.2.2 Threadless stems
|Steering column outer diameter (and inner diameter for the attachment on the stem
|measurement units: mm (inches)
||One inch standard
|28.6 ("1 1/8")
||1-1/8 inch standard
|31.8 (1 1/4")
||1-1/4 inch standard
|38.1 (1 1/2")
||1-1/2 inch standard
Table from: https://bike.bikegremlin.com/3729/bicycle-stem-size-standards/#1.2
Edit: Another scenario that came to my mind where the handlebars could be sliding around is if the steerer column / steerer tube of the fork is cracked, bent or damaged. The stem could be cracked or damaged as well, causing this. I would remove the stem and inspect for any damage.
This video shows how to remove and reinstall the stem:
Be careful about problems on the front end side of the bike. Any damage to the stem or fork is also a huge safety risk. As the frontend is responsible for steering and keeping balance, it can easily cause a bad crash.
Easiest is to take the bike to a local bike shop and they should diagnose it pretty quickly.