MySQLi and LONGTEXT Data Type
I ran into a similar problem this week using the MySQLi extension. Whenever my recordset returned data from a field that was defined as LONGTEXT data type, the data showed up in part in the Recordset dialog's Test window but nothing would appear on the page or in live preview. Fortunately for me, changing those fields to TEXT datatype worked since none of them exceeded 65,535 characters.
I did some further research and came across this article in Stackoverflow which might help others:
Perhaps Ray's team can add some code that checks to see if the field being queried is of type LONGTEXT or MEDIUMTEXT and if this is true, the necessary Store Results can be added.