This looks like a bug, in any case I can't find any explanation as to why. There's a test case here: http://sole.github.io/test_cases/css/range-thumb-pointer/
Works fine for me on Linux. On which platform did you see this problem?
Oh sorry, I forgot to mention the platform. It was in a system running Mac OS 10.10.5, and Firefox Nightly.
Does it work in older versions?
I don't have more versions handy, but it doesn't work in Developer Edition 45.
And it doesn't work in 43.0.4 on Mac OS X 10.9.5