Indesign data merge glitch

I have a 2 page indesign A3 landscape document which has 20 data fields (10 on each page) which are populated with numbers from a text file. I am having a really weird problem.  If I create a merged Indesign document it all works fine, but as it is a 3000 record data file it takes a long time to create this file.

But if I create a merged pdf from the original Indesign file, one of the datafields just repeats the number from the first page. All the other 19 fields merge correctly. I have recreated the data field but same happens.

I attach a merged pdf of records 1-20 to show the issue.

Any help appreciated!