DMT
Not sure it’s a common fault, but there are a few American threads saying it’s happened to them, just popped out.
I couldn’t find any reason for mine doing it, it was a freezing evening but everything had warmed up by then.
The breather is clear, the right amount of oil, strangely I had checked it that week. The seal is not a really tight fit in the housing but tight enough I would have thought.