I've had this problem a few times with a couple different Recteq grills. One is a RT-700 (Bull), the other is the same as yours, the RT-590.
I worked directly with Recteq support, and got my issues fixed. I've learned of a couple problems you can look into. One is once your grill reaches set temp, it should go into minimum feed rate. You can hear the grill slow down. The fan goes intermittent -- cycling on and off about once every three seconds or so, and you can hear the auger run a bit.
Two things -- if it doesn't do this, I would call or write Recteq support and describe the situation. They should send a new controller. It absolutely has to go into this mode once you reach set temp or just after if it continues to rise in temperature. I should state a caveat -- at higher temperatures it might continues to run more fully to maintain the heat. It shouldn't do that if it is over the set temp.
My situation was it was continuing to run full on. Like 100 degrees over and still sounding like a jet engine. It was to the point where I received the notification that my grill was overheating! Crazy high. I wonder if you got that.
If it does go into minimum feed, but the temperature continues to raise, check the Minimum Feed Rate setting on the app. It's under advanced settings. If it's the default setting of 3.5%, I've found a couple times when this is too high. You can lower it and see if that helps. The advise I was given from the support team was that I could lower it to 1% if I wanted. I tried 1.5% and it fixed the issue. I wouldn't expect that at 325; however, it is possible. I noticed these issues at lower setting -- 225 and lower.
You are using high quality pellets, so they burn hot and likely won't flame out even at 1.0% but I suggest trying 1.5%
If neither of these things fix your grill, call or write Recteq for suggestions. The grill is advertised as capable of staying within a couple degrees of set temp, and it really does when it's working correctly.
I hope you get this resolved and move on to enjoying the beauty of grilling with Recteq! You should get awesome results.