User parameter items that defined as data type "Text" are truncated to 64KB of data (in history), although documentation says that user parameters are eligible to return up to 512KB data, and the data type "Text" claimed as unlimited length text.

How repoducible: create any user parameter that returns value longer than 64KB, create an item with data type "Text", assigned to this user parameter key.
After the value is obtained and stored to the database, retrieve the history value using zabbix API (using method "history.get") or any other method capable of retrieving full item value (web interface is not suitable because it shows truncated value itself).
The value returned is truncated to 64KB of data. Futhermore, digging the zabbix database shows that table holding history values for "Text" items is defined as

"CREATE TABLE history_text (id bigint(20) unsigned NOT NULL, itemid bigint(20) unsigned NOT NULL, clock int(11) NOT NULL DEFAULT '0', value text NOT NULL, ns int(11) NOT NULL DEFAULT '0' ...". 

And we see that column "value" has MySql data type "Text" which is limited to 64KB of data according to MySql documentation.
Although it is easy to change this column type to a larger data type like "mediumtext", it still fills only up to 64KB maximum.
User parameter value tested on zabbix agent with zabbix_agentd -t <key_name> shows full length value returned by user parameter script, so agent itself is not the cause of the issue.