I found this comment in MoreLikeThisParams.java:
public final static String DOC_COUNT = mlt.count";
In other words, you're SUPPOSED to use &rows=n with the /mlt handler rather than &mlt.count=n.
So, it's not really a bug per se, more of an inconsistency that could be improved. For example, maybe the default for &rows in the mlt handler should be &mlt.count.
And maybe the doc could be improved a little. I would note that mlt.count is ONLY listed under the MLT search component and NOT mentioned in the handler. The doc for the handler shoulder probably explicitly say that it uses &rows rather than &mlt.count.