C# BulkCopy only sometimes supports unsigned bigint fields

We're trying to bulk copy into a table with an unsigned bigint column and sometimes it works and sometimes it doesn't

I've created a variety of tables and BCP-ed into them.

Here are the tables and whether they work,

Table : lock scheme AllPages. BCP WORKS

UId int

BId smallint

Line int

PID int

PDate datetime

PV varchar(255)

ES unsigned bigint

Table : lock scheme DataRows. BCP FAILS

UId int

BId smallint

Line int

PID int

PDate datetime

PV varchar(255)

ES unsigned bigint

(Change type to signed bigint)

Table : lock scheme DataRows. BCP WORKS

UId int

BId smallint

Line int

PID int

PDate datetime

PV varchar(255)

ES bigint

(Make unsigned bigint not the last column)

Table : lock scheme DataRows. BCP WORKS

UId int

BId smallint

Line int

PID int

PDate datetime

ES unsigned bigint

PV varchar(255)

It seems to be that unsigned bigint columns work so long as its not the last column.

Is this a known limitation/bug ?

Is there a fix ?