I've just retested it, and it does work with 'Sunday' (I reused the one used for Get Week) is passed to Week Dates/<First Day>.
Please recheck your modeling. Perhaps there's some other issue which causes Week Dates to execute before <First Day> (which is non-mandatory) receives the value. Try setting the trigger as mandatory.
Here's a screenshot of the model I tested: (note that non-mandatory Week Dates/<First Day> did work as well)
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