I just noticed this. Very large UDTs will create odd statistics, and may not be counted correctly by the Logix driver. You should avoid UDTs much larger than the connection buffer size.
It is vitally important that everything in a single tag in the PLC be scanned at the same pace by the tag groups, or you will get multiple requests for the same UDT to satisfy different parts of it.
You should read all of this topic and the cross linked topics: