In brief, Yueh is a person who is supposedly above any consideration of malice or duplicity toward his patients, the house Atreides, because of the conditioning that Doctors from the Suk School receive.
IMPERIAL CONDITIONING: a development of the Suk Medical Schools: the highest conditioning against taking human life.
There's no point investigating him as a potential traitor beyond confirming that he's the real Yueh.
“Hawat will have divined that we have an agent planted on him,” Piter said. “The obvious suspect is Dr. Yueh, who is indeed our agent. But Hawat has investigated and found that our doctor is a Suk School graduate with Imperial Conditioning — supposedly safe enough to minister even to the Emperor. Great store is set on Imperial Conditioning. It’s assumed that ultimate conditioning cannot be removed without killing the subject. However, as someone once observed, given the right lever you can move a planet. We found the lever that moved the doctor.”
Even when it becomes blindingly obvious to anyone with eyes that Yueh must be the traitor, Paul still can't understand what's happening. He thinks he's mistaken, or that it's some manner of ruse.
He wondered who could’ve captured her. His own captivity was plain
enough– to bed with a capsule prescribed by Yueh, awaking to find
himself bound to this litter. Perhaps a similar thing had befallen
her. Logic said the traitor was Yueh, but he held final decision in
abeyance. There was no understanding it–a Suk doctor a traitor.