You need to transfer the Earpiece Speaker and Front Sensor Assembly from the original screen to have face ID as they are paired to the logic board. I would transfer your camera assembly as well.
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'''iPhone XS Max Earpiece Speaker and Front Sensor Assembly Replacement'''
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[https://www.ifixit.com/Guide/iPhone+XS+Max+Earpiece+Speaker+and+Front+Sensor+Assembly+Replacement/113638|'''iPhone XS Max Earpiece Speaker and Front Sensor Assembly Replacement''']
You need to transfer the Earpiece Speaker and Front Sensor Assembly from the original screen to have face ID as they are paired to the logic board. I would transfer your camera assembly as well.
'''iPhone XS Max Earpiece Speaker and Front Sensor Assembly Replacement'''