Pandoc recognizes an abbreviation and inserts a nonbreaking space (among other things, this prevents a sentence-ending space from being inserted in LaTeX output).

% pandoc -t native
Mr. Bob
^D
[Para [Str "Mr.\160Bob"]]

Here pandoc readjusts the softbreak so that the nonbreaking space can be inserted:

% pandoc -t native
Hi Mr.
Bob
^D
[Para [Str "Hi",SoftBreak,Str "Mr.\160Bob"]]

If you don't want this to happen you can escape the period:

% pandoc -t native
Hi Mr\. Bob
^D
[Para [Str "Hi",Space,Str "Mr.",Space,Str "Bob"]]