In reply to OwenM:
It's a design flaw of the soft touch smooth coating which seems to break down over time. My paragliding helmet from a few years back has done the same thing. I had some limited success wiping off the sticky degraded coating with meths but it's messy and wasn't totally successful so I now ignore it and have bits of grass stuck to it.
Some searching around google may result in other possible solutions, it was a common problem in paragliding circles a few years ago, I'm sure it's an identical problem.
Here is a random link I found which describes the problem.
https://midlifeminutes.com/why-do-ski-helmets-get-sticky/