Even more baffled now. Did more testing.
#1 Create a new form with just two fields and created new form connections for doing updates to submitted forms – it worked!
#2 With the new form added another field, hide it and mapped the published version and again tried the update connection with the hidden field – it worked!
Used default permissions.
The other form which has many more fields and in this case was a field added to already submitted forms and was mapped (hence the reason for test #2). There is no events on this field.
Very confused on where to go from here.