This may be due to the maxshowndata runtime parameter, which limits the size of stream item data that is displayed directly in API responses. You can change this parameter or use getttxoutdata to get the stream item data in full.
If this does not explain the problem, can you please post the API command you are running and the output you receive? You can chop out the middle of the long hex string representing the data item so it is readable in this Q&A forum.