It's not by any chance a 'listening' datagrid is it? If so there is an issue with displaying text area fields that I reported back in December (ticket 204933). Here's a sample form where the widget on the right is a listening datagrid and it contains the same field in a normal text box and a text area for testing...
The error is shown in the screenshot below. The data in the text area version of the Summary field is incorrect and does not update
As I say, this was reported against 4.8.9 in December, but the response from Support was to accept the issue but...
'We have analyzed the ticket and would like to inform you as follows. Your ticket has not been scheduled for a Release within the next 2 months.'
I have also reported a similar but an even worse issue in listering datagrids in version 5 in January (ticket 205321). I consider displaying incorrect data to application users to be a critical issue, but apparently Mendix does not. I am still waiting for fixes to these issues nearly 4 months on...
I am told the version 5 issue should be fixed in the next release, but as far as I know there are no plans to fix the v4 issue.