At only 300 yards, a whole lot of factors matter very little if at all - among these "don't matters" are air temperature, barometic pressure, humidity, rifle cant, ammo/powder temperature, wind speed and angle, batch of bullet, lot of powder, brand of primer, loose bedding screws, etc.
If you needed 3.5 MOA (up from 100y) all summer long, but 4.5MOA recently, that is a *big* change, one that is unexpected and not easily explained. And if one day in July you only needed +1.5 MOA to go from 100y to 300y, that's a ridiculous outlier that sounds alarm bells. There is something wrong in your setup somewhere, something that would be worth while discovering and fixing.
From one shooting session to the next, it's completely reasonable to see your 300 elevation differ by a quarter minute from your usual setting; click it in if it needs it and don't even spare it a second's thought. It's even within the realm of reasonableness to see a half-minute different in your elevation setting from your long term average, but any more than that I would declare that there is something wrong somewhere, which needs to be found and fixed.
The prime culprit is always the scope, if you are using one (or its mounting system). Start by verifying that the rings and bases are tight. If you have a collimator (optical boresighter), put that on, click your scope around, and see if the scope's internals move as they should - cleanly, without backlash or hesitation, in consistent and proper amounts. If you don't have a collimator but if you do have another known good scope that you can put on your rifle, do that and see how things work with another scope.