Can you please try enabling "Apply NULL Values" on the Middle Name on your Base Object. This should make a NULL value with higher trust to be reflected in the Merged record.
Thanks for your response. In my case, I would still want to pull in a valid Middle Name from any of the other trusted sources like the second or third trusted source if they have a Middle Name when the Middle Name is Null in the highest trusted source. I just don't want the Middle Name from the source with the wrong Middle Name to win. Is there a way I can exclude the source with the wrong Middle Name (first entry in the screenshot) during the trust calculation?
Yes. You can set trust score to 0 for max and min to achieve it. Other way is downgrade the trust using validation rules for this particular source.
Thank you I will try setting the trust score to 0 for max and min values and see if it works.