Linq Query returns 0 results, but should return 1

Question

I have a SQL Server Compact Edition version 3.5 for a winforms app that is used as to store order information, but I have had a complaint from someone who is having issues where the detail isn't showing up after it has been finished and submitted, and by receiving the database of the user I can reproduce the problem, but I cannot figure out why. What I'm trying to figure out is why they aren't syncing up, because others do work just fine, and I have searched Stack Overflow and not found an answer that has worked for me.

The results of linqpad/SSMS against the database I received are the same and bring up one row as a result of the following query (same query as entity framework profiler shows):

SELECT [Extent1].[Id]                  AS [Id],
       [Extent1].[OrderHeaderId]       AS [OrderHeaderId],
       [Extent1].[Price]               AS [Price],
       [Extent1].[Quantity]            AS [Quantity],
       [Extent1].[OverridePrice]       AS [OverridePrice],
       [Extent1].[ShippingWeight]      AS [ShippingWeight],
       [Extent1].[ExtendedPrice]       AS [ExtendedPrice],
       [Extent1].[OrderId]             AS [OrderId],
       [Extent1].[ProductItemNo]       AS [ProductItemNo],
       [Extent1].[ProductItemNoTypeId] AS [ProductItemNoTypeId]
FROM   [OrderDetail] AS [Extent1]
WHERE  [Extent1].[OrderHeaderId] = 'eec06164-a052-4c23-9575-8fe1b80c8baa' /* @p__linq__0 */

However, if I use either of the following statements I get no results:

_orderDetails = ctx.OrderDetail.Where(o => o.OrderHeaderId == _order.Id).ToList();

_orderDetails = (from od in ctx.OrderDetail
                 where od.OrderHeaderId == _order.Id
                 select od).ToList();

Order Header:

-- Creating table 'OrderHeader'
CREATE TABLE [OrderHeader] (
    [Id] uniqueidentifier  NOT NULL,
    [PONumber] nvarchar(4000)  NULL,
    [InternalOrderText] nvarchar(4000)  NULL,
    [TimeStamp] datetime  NOT NULL,
    [IOCTrackingNo] nvarchar(4000)  NULL,
    [CarrierCode] nvarchar(4000)  NULL,
    [HowEntered] nvarchar(4000)  NOT NULL,
    [MessageName] nvarchar(4000)  NOT NULL,
    [RepID] nvarchar(4000)  NOT NULL,
    [BatchID] nvarchar(4000)  NOT NULL,
    [Status] nvarchar(4000)  NOT NULL,
    [ApplicationVersion] nvarchar(4000)  NOT NULL,
    [CustomerAccountID] nvarchar(4000)  NOT NULL,
    [CustomerSubAccountID] nvarchar(4000)  NOT NULL,
    [SystemArrivalDate] datetime  NULL,
    [OrderId] int  NOT NULL
);
GO

-- Creating primary key on [Id] in table 'OrderHeader'
ALTER TABLE [OrderHeader]
ADD CONSTRAINT [PK_OrderHeader]
    PRIMARY KEY ([Id] );
GO

Order Detail:

-- Creating table 'OrderDetail'
CREATE TABLE [OrderDetail] (
    [Id] uniqueidentifier  NOT NULL,
    [OrderHeaderId] uniqueidentifier  NOT NULL,
    [Price] decimal(18,2)  NOT NULL,
    [Quantity] int  NOT NULL,
    [OverridePrice] bit  NOT NULL,
    [ShippingWeight] decimal(18,3)  NOT NULL,
    [ExtendedPrice] decimal(18,2)  NOT NULL,
    [OrderId] int  NOT NULL,
    [ProductItemNo] nvarchar(4000)  NOT NULL,
    [ProductItemNoTypeId] uniqueidentifier  NOT NULL
);
GO

-- Creating primary key on [Id] in table 'OrderDetail'
ALTER TABLE [OrderDetail]
ADD CONSTRAINT [PK_OrderDetail]
    PRIMARY KEY ([Id] );
GO

Foreign Key:

-- Creating foreign key on [OrderHeaderId] in table 'OrderDetail'
ALTER TABLE [OrderDetail]
ADD CONSTRAINT [FK_OrderDetailOrderHeader]
    FOREIGN KEY ([OrderHeaderId])
    REFERENCES [OrderHeader]
        ([Id])
    ON DELETE CASCADE ON UPDATE NO ACTION;

-- Creating non-clustered index for FOREIGN KEY 'FK_OrderDetailOrderHeader'
CREATE INDEX [IX_FK_OrderDetailOrderHeader]
ON [OrderDetail]
    ([OrderHeaderId]);
GO

Answer 1

Myself and several others weren't able to figure out why the link became broken, so I decided to change the database to SQL CE 4.0 because of the ability to use int identity fields, and we haven't had this issue since.