EF Update Statement is Violating a Unique Index - sql

Question

I've got a bit of an issue which has exposed my lack of understanding of Uniqueness Constraints. This is the table in question and there is a unique, non-clustered index on the DataManagerId, SurgeonId and HospitalId columns of a table, as shown in this image (apologies, I don't have Visio):
I am using Entity Framework ("EF") and it has generated an UPDATE statement akin to this:
exec sp_executesql N'UPDATE [dbo].[DataManagerSurgeon]
SET [DataManagerId] = #0, [SurgeonId] = #1, [HospitalId] = #2, [DateAdded] = #3, [DateRescinded] = #4
WHERE ([Id] = #5)',N'#0 int,#1 int,#2 int,#3 datetime2(7),#4 datetime2(7),#5 int',#0=24,#1=221,#2=10001,#3='2018-01-02 08:58:29.1061413',#4='2018-01-02 08:58:29.1061413',#5=122
I'll clean that up a bit for the purposes of this question:
UPDATE [dbo].[DataManagerSurgeon]
SET [DataManagerId] = 24, [SurgeonId] = 221, [HospitalId] = 10001, [DateAdded] = '2018-01-02 08:58:29.1061413', [DateRescinded] = '2018-01-02 08:58:29.1061413'
WHERE [Id] = 122
The problem is, this is seeking to alter data (hence the update statement). And there is already a row with the combination (24,221,10001) for (DataManagerId, HospitalId,SurgeonId). As a result, the database is having none of it:
Violation of UNIQUE KEY constraint 'UC_ManagerSurgeonHospital'. Cannot insert duplicate key in object 'dbo.DataManagerSurgeon'. The duplicate key value is (24, 221, 10001).
As further background, I am only trying to update the DateRescinded column and if I were to write the statement manually, it would look like:
UPDATE [dbo].[DataManagerSurgeon]
SET [DateRescinded] = '2018-01-02 08:58:29.1061413'
WHERE [Id] = 122
I guess I learnt that you can violate a uniqueness constraint with an UPDATE statement, even when identifying the row by primary key in such a way that, at the end of the transaction, the conditions of the uniqueness constraint would still be met.
But my question remains, if EF is generating an UPDATE statement which violates a uniqueness constraint (index), how do I get around that? Note: the least optimal solution would be writing manual SQL and sending that to the database. It can be done, but defeats the purpose of using an ORM like EF.
Also note that I added the following code in the Mapping file for that table to make EF "aware" of the index:
this.HasIndex(t => new { t.DataManagerId, t.SurgeonId, t.HospitalId })
.IsUnique(true)
.IsClustered(false)
.HasName("UC_ManagerSurgeonHospital");
Final addendum - I have been referring to this as an Index and I'm not sure how SQL Server distinguishes these from CONSTRAINTS, but the create statement for such an Index looks like:
ALTER TABLE [dbo].[DataManagerSurgeon] ADD CONSTRAINT [UC_ManagerSurgeonHospital] UNIQUE NONCLUSTERED
(
[DataManagerId] ASC,
[SurgeonId] ASC,
[HospitalId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
I only mention that because people might ask why I implemented it as an Index and not a CONSTRAINT. But apparently, this is how you do multi-column constraints in SQL Server.
Thanks