My two cents on the matter.. I bought a Pro 34 over 4th of July from Home Depot and experienced the same symptoms as you, it was cooking super slow and I knew something was wrong. I ended up putting a bunch of my own probes inside the unit and checking the readings from the Traeger controller and my own. When it first started up everything was fine but as temps rose the differences started to show themselves. On the high end of the dial the Traeger was reading 100f OVER what the actual temps were.
I ended up calling them to say something was broken and their response was no, all of my other probes are broken and only theirs is correct. Didn't matter what I said they just told me I was doing it wrong and the unit was fine. I knew this was false because it got to the point I had multiple probes in the same position as their RTD and saw the differences. I decided to follow their chicken win recipe and instead of it taking 35 minutes on high the wings took an hour and a half to come up to safe temperatures.
They sent me a survey some time after the support phone call asking how they did and I blasted them on it. This prompted them to call and email me back right away asking what was wrong. I spoke to them and told them it was unsafe to cook on the grill using their own recipes and guides because the 100 degree difference. Eventually they agreed to send me a new RTD. It came, I replaced it, and it fixed the issue.
I then also decided to upgrade the controller to a PID and it made a world of difference. Just did 14 pounds of pork butt on Monday and the Pro 34 held temps like a champ, was perfectly dialed in to what temps I set the controller to, and now I have more pork than I know what to do with in my fridge. That's a completely separate topic but the PID controller is amazing and I should have bought one to upgrade my old unit years ago.
I'd say get a third party meat probe and wedge it right up against the RTD the unit came with and plot the differences. Follow what support is going to tell you, which is turn it on smoke and let it sit for 15 minutes. Then turn it to low and wait for 10 to 15, then medium waiting the same, and then high. See if you get the same difference in readings. When the grill said 400 my probes all read 300. When I opened the lid to look at them all the temp would drop and the unit would say it was at 330 or 340 while mine read 230 or 240.
Did you just get yours too? Maybe they had a bad batch on the production lines.