I've gone through 3 or 4.

The lever itself is softer than the splines, so it wears first. After warranty return after return I gave up and had a look at the lever.

Firstly the bolt hole is tapped on both sides, so it will never close up as you tighten it, the gap is pretty much fixed in line with the threads.
Second, the lever has groves in line with the splines, so you're only ever going to get any purchase on a small part.

Accepting that there's not alot to be done, I drilled out the bolt hole and put a long bolt and a locknut on the end.

I closed up the spline hole in a vice then refitted it to the bike. I had to push a screwdriver blade into the small gap to open it enough to remount it.

Put the bolt through and tightened it. The lever was nice and stable.

....

...

...

for about a week !

On the plus side it hasn't gotten any worse years later.

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Never underestimate the power of stupid people in large groups.