You'll need to describe your layout and the underlying tables/ relationships in much more detail. Also, you have posted in the FM GO forum. Is this a question about using FM GO on an iPad, iPod or iPhone or are you asking a question about FileMaker Pro on a windows or macintosh computer?
The problem shows in testing on both FM Go 13 and FMPro 13. Target environment is FMGo.
The question is why would a text field require a number in it to be saved?
To repeat: You'll need to describe your layout and the underlying tables/ relationships in much more detail.
What you describe might happen if, in manage database, your field is defined to be of type number. It is not typical of a field of type text so if this is happening in a text field, we need more information in order to determine why this is happening in your case.
It sounds like the key field on the other side of the relationship is a number field. The type of field on both sides of the relationship should match to work properly.