Morning Frans.
My project doesn't use entity inheritance. It would appear that the cause of my issue is different to the one that you've described as when tracing the server traffic from my test solution (using the original code) I can see that the FetchEntityCollection returns the first 20 Activity records and the FetchExcludedFields returns all 20 matching Description values in the same order.
exec sp_executesql N'SELECT DISTINCT TOP 20 [dbo].[Activity].[recID] AS [RecId], [dbo].[Activity].[subject] AS [Subject],
[dbo].[Activity].[location] AS [Location], [dbo].[Activity].[probability] AS [Probability], [dbo].[Activity].[quoteReference] AS [QuoteReference],
[dbo].[Activity].[estimatedRevenue] AS [EstimatedRevenue], [dbo].[Activity].[startDate] AS [StartDate], [dbo].[Activity].[endDate] AS [EndDate],
[dbo].[Activity].[completeDate] AS [CompleteDate], [dbo].[Activity].[duration] AS [Duration], [dbo].[Activity].[dayeventflag] AS [DayEventFlag],
[dbo].[Activity].[privateFlag] AS [PrivateFlag], [dbo].[Activity].[typeID] AS [TypeId], [dbo].[Activity].[priorityID] AS [PriorityId],
[dbo].[Activity].[directionID] AS [DirectionId], [dbo].[Activity].[stateID] AS [StateId], [dbo].[Activity].[statusID] AS [StatusId],
[dbo].[Activity].[regardingTypeID] AS [RegardingTypeId], [dbo].[Activity].[regardingAccountID] AS [RegardingAccountId],
[dbo].[Activity].[regardingContactID] AS [RegardingContactId], [dbo].[Activity].[regardingProspectID] AS [RegardingProspectId],
[dbo].[Activity].[reminderFlag] AS [ReminderFlag], [dbo].[Activity].[reminderDate] AS [ReminderDate], [dbo].[Activity].[createDate] AS [CreateDate],
[dbo].[Activity].[createUserName] AS [CreateUserName], [dbo].[Activity].[modifyDate] AS [ModifyDate], [dbo].[Activity].[modifyUserName] AS [ModifyUserName],
[dbo].[Activity].[ownerUserName] AS [OwnerUserName] FROM ( [dbo].[Activity] INNER JOIN [dbo].[ActivityParticipant]
ON [dbo].[Activity].[recID]=[dbo].[ActivityParticipant].[activityID]) WHERE ( ( [dbo].[ActivityParticipant].[userName] = @UserName1))',
N'@UserName1 nvarchar(50)',@UserName1=N'chris.hilton'
exec sp_executesql N'SELECT TOP 250 [dbo].[Activity].[recID] AS [RecId], [dbo].[Activity].[description] AS [Description]
FROM [dbo].[Activity] WHERE ( ( ( [dbo].[Activity].[recID] IN (@RecId1, @RecId2, @RecId3, @RecId4, @RecId5, @RecId6, @RecId7, @RecId8,
@RecId9, @RecId10, @RecId11, @RecId12, @RecId13, @RecId14, @RecId15, @RecId16, @RecId17, @RecId18, @RecId19, @RecId20))))
ORDER BY [dbo].[Activity].[recID] ASC',N'@RecId1 uniqueidentifier,@RecId2 uniqueidentifier,@RecId3 uniqueidentifier,@RecId4 uniqueidentifier,
@RecId5 uniqueidentifier,@RecId6 uniqueidentifier,@RecId7 uniqueidentifier,@RecId8 uniqueidentifier,@RecId9 uniqueidentifier,
@RecId10 uniqueidentifier,@RecId11 uniqueidentifier,@RecId12 uniqueidentifier,@RecId13 uniqueidentifier,@RecId14 uniqueidentifier,
@RecId15 uniqueidentifier,@RecId16 uniqueidentifier,@RecId17 uniqueidentifier,@RecId18 uniqueidentifier,@RecId19 uniqueidentifier,@RecId20 uniqueidentifier',
@RecId1='5D528122-EE27-DD11-A2B7-0019D1462DD5', @RecId2='75528122-EE27-DD11-A2B7-0019D1462DD5',@RecId3='6D528122-EE27-DD11-A2B7-0019D1462DD5',
@RecId4='5F528122-EE27-DD11-A2B7-0019D1462DD5',@RecId5='77528122-EE27-DD11-A2B7-0019D1462DD5', @RecId6='6F528122-EE27-DD11-A2B7-0019D1462DD5',
@RecId7='A5097188-ED27-DD11-A2B7-0019D1462DD5',@RecId8='79528122-EE27-DD11-A2B7-0019D1462DD5', @RecId9='A7097188-ED27-DD11-A2B7-0019D1462DD5',
@RecId10='61528122-EE27-DD11-A2B7-0019D1462DD5',@RecId11='63528122-EE27-DD11-A2B7-0019D1462DD5', @RecId12='65528122-EE27-DD11-A2B7-0019D1462DD5',
@RecId13='57528122-EE27-DD11-A2B7-0019D1462DD5',@RecId14='67528122-EE27-DD11-A2B7-0019D1462DD5', @RecId15='59528122-EE27-DD11-A2B7-0019D1462DD5',
@RecId16='71528122-EE27-DD11-A2B7-0019D1462DD5',@RecId17='69528122-EE27-DD11-A2B7-0019D1462DD5', @RecId18='5B528122-EE27-DD11-A2B7-0019D1462DD5',
@RecId19='73528122-EE27-DD11-A2B7-0019D1462DD5',@RecId20='6B528122-EE27-DD11-A2B7-0019D1462DD5'