Today it became 50 as it should be. But we are now on the evening of day 4 of a busy week with lots of posts in multiple threads. If the problem persists, the count will go down over the weekend again. Will report after the weekend. Maybe the number in the cache (Post 17) is the cause and solution at the same time.
My humble theory, maybe it helps:
The Cache must be big enough, to contain at least the requested number as different threads (topics as you call it).
So if one sets the widget to 50 and has the cache at 100, it would only work, if most of the threads have just 1-2 posts.
Now lets imagine, the cache is 100, but the last 80 posts were part just of *one* thread (topic). And before that, there were 20 posts in another thread.
In that scenario, there would be just 2 entries displayed in the widget, as the cache contains 100 posts, but just 2 threads!
Just a theory.