It’s hard for me to say – I only see part of the shortcode function, so it would help if you can paste the entire thing or email it to me. The default position setting only applies to ‘mappress’ shortcode, so that’s probably not the issue.
Is it possible the mastermap shortcode is echoing its output rather than returning it? You might try just returning a string like ‘mastermap here’ from the shortcode, with nothing else, to see that the shortcode output is positioned correctly without a map.
You might also want to look at the ‘mappress’ shortcode in file ‘mappress.php’ for an example of how the maps are output by default.