So helped install a replacement exterior door knob/lock recently. They bought a cheapish brinks unit. Old one came out fine. New one went in fine I would say. Everything works as it should. But when the key is inserted you can freely turn it 360, round and round etc. You can lock in manually or with the key. But why does it turn freely when the key is inserted. Sump'in ain't right

It sounds like the tailpiece has a problem.

Something might be broken or it may just be installed wrong

Take it back apart.

Make sure you follow the directions.

Some locks must be installed with the bolt open to start. Some tailpieces must be oriented before installing the lock.

I am guessing it is one or both of the bolt or tailpiece oriented incorrectly during the installation.

I agree that something may be broken.
I’m not familiar with that brand but on the ones we install there are two “prongs” that are inserted into the bolt section thereby locking it into place.
Maybe your’s are broken/missing.

My input may not help, but I have an old house with old doors and locks. Had the local locksmith come out to try to fix the same issue with a Schlage (I believe) lock.

How it turned out, some interior part of the lock was a couple of millimeters short of connecting to the other side. Lock is old enough that he would have to machine the part.

Not worth the effort in my case as the problem was from the exterior side.