Hey welcome to AGN Sparker!
The Avenger is one of the best airguns on the market now, especially for the price. Glad you got one! Let us know if you have any questions at all. Lots of us have one on AGN.
So....a lot of the Avengers have "regulator creep" and it sounds like exactly what you are experiencing. I've had mine since December '22 and I've set my reg set from 1.6k, crept to 1.8k then up to about 2.2 or 2.3k now and each morning after shooting it'll climb to 2.6k or so.
It's a common thing but some people will just tell you to dry fire like 2 shots and then go out and shoot cause the dry firing will lower the plenum pressure to what your reg pressure is set to. That's what I do.
Since yours is going from the air tube to the plenum, that eliminates an external leak (good). You can try the silicone in the fill port and see if that helps. It is also a new gun and can take a bit to settle in. Also, if you fill with a compressor the air will be warmer and the psi will be higher. In addition, if you are pumping up inside in AC then take it outside in the sun the psi will rise. Laws of physics etc. BUT from what you say it sounds like reg creep.
There are things you can do to it to help settle it like degas, punch out the pin that is in the block over the reg screw and unthread the screw out and you'll see the delrin? material on the end. You can lay some 2,000 grit or more sandpaper on glass and flatten that seat flush to make it more consistent in pressure. However since yours is new and you're also new to airguns and PCP idk if I'd recommend that yet.
Before
After sanding
And here you can read up on what else I did but I've modded mine...a lot:
Polish regulator washers, etc but again I don't think you should be taking it apart since it is under warranty.
Even after doing all that to mine it still creeps though but my standard deviation is low and it's very accurate gun, like 18 shots of 18.13 JSB hitting a 2 inch spinner at 75 yards.
Shot string:
FPS 935.7, 934.5, 930.3, 931.7, 930.3, 934.5, 928.9, 935.9, 928.9, 930.3, 927.5, 928.9, 930.3, 924.8, 924.8, 927.5, 933.1, 927.5
Ave FPS: 930.3
Ave FPE: 34.85
ES: 11.1 FPS
Std Dv: 3.2956199888809
Anyway all that was to say it's accurate even with the overnight 200psi creep. If yours creeps that much in like a few hours or faster (but not due to just filling up or ambient temps) then I'd consider warranty swap. But if its overnight and you still have warranty time left I'd shoot a whole tin 500 pellets through it and see where you're at after the silicone oil.
Im saying this for general reference since youre new to PCP, not because "pro tune" is bad or anything but if you tune to 95-97% of max velocity at a reg setting (so go from 0 hammer spring to all the way in then see what your FPS is on a chronograph, shooting a few at each 1/2 HS turn) then go back to whatever is 95-97% of the max velocity, you will be at a efficient spot for air use, and also for consistency in psi. You will also be more accurate even if the psi is off a bit because of the 95-97% tune. You can search on AGN for "tune for the knee" on a shot curve for more explanation.
Accuracy is king in airgun world. Much better to be slow and hit what you aim at than go for highest FPS. All you'd do is miss faster lol