Since you describe it as a constant modifier rather than a triggered ability like all of the similar cards i could find i would say this sounds like a Layers deal. Layers are the order in which constant effects are applied and I personally don’t know enough to say for sure, but if you start by looking up MTG Rules Layers that should be a solid starting point.
Right, thats likely why all of the similar cards i could find on a search were triggered abilities. But, if you go through all of the layers in order then the resulting power after 1 pass will stay that value until something else changes the game-state and causes them to recheck since the +X/+0 will have already been applied.