Version: Indesign CC 2017.1. Same behaviour in both Windows 7 and El Capitan 10.11.6.
Steps to reproduce: Create a layout file and a tab delimited text file like the ones seen on the info images.
Expected result: A pdf with the same data as the text file and preview.
Actual result: A pdf with its first record correct, but the following two incorrect. See description image.
Decrease the column names number of characters and/or make sure no placeholder has overset text (used solution).
Create a merged document. The values seem to work as expected then (this solution was not possible in my case).
Although making sure there are no overset text frames seems to fix the problem there were many text frames like that in the actual layout, and all except one of them worked as expected. The data itself fit the frames without any problem and no overset warning is shown after the export.
Provided are the actual test files as well. Se file ”datamergebug_files.zip”.
A really nasty bug since the preview should definitely not differ from the actual output.
Colin Flashman commented
This is an issue I've experienced over the years with Data Merge - when a placeholder is overset, exporting to PDFs can produce faults like the user has experienced. The fix is to make sure the placeholder has no overset text by either making the text frame bigger, using the auto-size frame fitting, or using a GREP style to make a very tiny font when displaying the preview text (i.e. apply a GREP style applying to double-chevron items that is 1pt in size). However, it would be great if this solution was not required.