It does seem the hyphens can cause problems, there are a few mentions in the WordPress TRAC (Bug) database too.
For something like this, I suggest you just print out the results of your query using a PHP echo statement first. That will at narrow down whether this has anything to do with MapPress or if it’s just a query issue.
Assume your the query you normally use is in $query, you could put this in your template:
<?php $wpq = new WP_Query($query); print_r($wpq); ?>
MapPress adds a few extra parameters. If you want to compare the above query to how MapPress runs it, try adding this too:
<?php $query = "map=true&post_type=any&cache_results=false" . $query; $wpq = new WP_Query($query); print_r($wpq); ?>