Looks like it is not being detected that a new sentence starts after a heading, if there is not an empty line.
But a short line without an ending punctuation could be detected IMHO. Example:
This is a header
Because it it a lot shorter than the next line, and there is no ending character like .!? and this line start with a capital (even though that is a trick characteristic because of proper names).
Because of that, the line will be rather short in letters. There is already a number on that in the rule, but the line break is ignored completely, unless there is an empty line between.