How to update a record Using SQL SERVER UPDATE without updating identity field

The process established by the company I work for is that we have the document coming into a MAPPER, that links the incoming fields to the SQL SERVER UPDATE Snap. This tells the UPDATE snap which fields should receive which values, but this includes the identity field. I cannot exclude the identity field, because then the update fails because it does not have a value to compare in the where clause

To put this into a SQL context:

What I want to do:
UPDATE table
SET FIELD = value
WHERE ID = id

What the Mapper + Update is doing:
UPDATE table
SET FIELD = value
AND ID = id
WHERE ID = id

And if I remove the ID field from the Mapper, then the pipeline fails because the field doesn't exist, but esentially I am trying to tell SQL to:
UPDATE table
SET FIELD = value
WHERE ID = null


This has only occurred on 1 of our servers, which is how I've discovered that the ID field is being updated on the other servers. We had assumed that Snap knew not to update the ID field.
I am investigating SQL to see how the other servers are able to allow this on IDENTITY fields
But I am posting here, because I am concerned that we are going about doing the updates, incorrectly

Thanks for any assistance anyone is able to provide