Shimano Ultegra Cassette Issue

I have Ultegra WH6800 rear wheel and just bought a new Ultegra 11 speed cassette CS-R8000 for it. When I put the cassette on, the smallest cog (11 tooth) goes on, but doesn’t sit down all the way on the free-hub body, so the lock-ring is not fully engaged (I can tighten it up, but only about 50% of the threads are used). the smallest ring sits about 2 mm higher than if it was snugged right down, which means that only about half of it is engaged on the Freehub splines. It’s definitely correctly assembled, no dirt etc or obstructions at base of the splines. No spacer inadvertently fitted

What is going on – should it always sit like this?

I wondered if it was a 10 speed freehub, but I’m pretty sure those Ultegra wheels only come in 11 speed configuration (they are 2 years old). Freehub does not have a part number stamped on it to check