Thank you for the post.
Development and Testing are aware of this issue. I've added your comments to the original report.
Another user also reported this here:
No additional information is available at this time.
As a workaround, Consider using OnLayoutKeystroke only to detect a "start of scan" character and then put the cursor in the correct text field to receive the scanned data. Then use a different trigger to process that data only after the scanner has input the full set of data from the barcode. We have a scanner configured to put a return character at the end of scanned text and use the OnObjectExit trigger to process the scanned data.
As far as I know, that approach continues to be functional in filemaker 12.