Commands character limit?
Apparently yes, because reasons

https://stackoverflow.com/questions/9607393/drupal-7-ajax-command-creates-unwanted-div

The solution to anyone else having this issue is to use either your own callback, or instead use ajax_command_invoke("#my-div", 'append', array($content)); and that will do the same as ajax_command_append, both solutions are more fleshed out in the link