Help - bike is sending me up the wall

[Eichhörnchen]

I've had to assemble a flat-packed bike (a trike) - shouldn't have been a problem for me even though I know next to nothing about bikes.

Problem is the handlebars. They came with a plastic guard over the spline which I took off, also a thin metal shim behind the spline which I also took off. This following the advice of another purchaser who'd left a review saying that the instruction book was completely inadequate for anyone lacking engineering experience, so he gave this advice freely and what's more offered to write up a proper assembly sheet for them (Temu).

Well I then slid the handlebar stem into the cycle's frame post, adjusted for height, then tightened the top as firmly as I could with the large allen key provided.

It tightened up as expected - but then as soon as I turned the handlebars I found them to be loose - the front wheel pointing straight ahead - the handlebars not gripping. When I removed them again and shone a light into the frame post it was completely smooth inside. Now surely there should be vertical grooves in there to engage with those on the handlebar spline? Is this a defective product??