“Porter actually received a telegram from Gideon Wells”
The telegram delivered to Powhatan was sent by Seward.
After a conference between Lincoln, Welles and Seward, Lincoln ordered Seward to recall Powhatan. Seward did as directed, but he signed the telegram “Seward” in stead of “by Order of the President”. Since Seward was not in Porter’s chain of Command and Porter was in possession of an order signed by the President. He chose to ignore the recall order by Seward.
Porter ignored it because it didn't say "Lincoln."