I made some experiments, and the problem definitely exists.
The results (length of the result, each time much smaller than expected): with reverseOrder=True revCount=500 -> 1393 revCount=5000 -> 500 revCount=50 -> infinite loop?
with reverseOrder=False revCount=500 -> 61 revCount=5000 -> 61 revCount=50 -> infinite loop?
I am not sure if the problem is in Pywiki or API, but without API I got another wrong result (997). Maybe something is hidden in http://en.wikipedia.org/w/index.php?title=Special:Log&page=Barack+Obama? Pending changes? I'll try to continue tomorrow, but I am afraid that I will miss admin rights. When I tried to get API results directly in browser, it crashed.
In huwiki it seems to work.