pandoc/test/command/md-abbrevs.md

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" ] ]

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

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