Mailed in document not saving in ND6

I have an R5 database which is now running on an ND6 server.

Users forward emails to it which they wish to be kept.

A button on the form switches forms for them.

Some of the documents do not save changes, whilst others do,

There is no SaveOptions field defined on the original form, but the switched to form has SaveOptions set to “1”.

If I use the same option on the same document from an R5 client, then it saves normally.

Can anyone give me any pointers?