Deadlock scenario, where is this S lock coming from?

I am encountering the below deadlock scenario. Please see the attached object definitions and execution plans. The plans were pulled out of cache from a system where this deadlock does occur.

I understand what all of this data is telling me but what I am confused about is why the spUpdateUserAccount stored procedure is taking an S lock on IX_Person_TaxPreparer_rowCreated. This lock appears unnecessary and there is no reference to IX_Person_TaxPreparer_rowCreated in the execution plan for spUpdateUserAccount.

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