Half the time ksyntaxhighlighting is not doing its job

I have the haskell highlighting file installed (through the download button).

Half the time after I start up Qt Creator, highlighting of haskell files is not done, even though I also do not get the "no highlighting file" warning. So it seems like the code knows that highlighting is available, but just doesn't highlight.
Re-opening files etc does not have any effect in that case - either highlighting works during that Qt Creator run, or it doesn't.

In a session with a haskell and a bash file open this effect happens independently for haskell and bash highlighting, i.e. I can have highlighting for both, only one of them, or none.