So apparently this is called the 'Fudd Flag', named after Looney Tunes' Elmer Fudd. A Redditor commented an explanation on why this was a thing in vintage cartoons. To quote their explanation:
It happens because animated backgrounds tended to be higher quality in order to establish a scene and mood despite the cartoony characters. Scooby Doo is more more serious in tone than Yogi Bear partially because the higher quality music and the backgrounds being more complicated.
The object that gets interacted with is made cheaper like the characters because its going to have to move and thus stay on-model as it does so, and because in cell animation its literally a transparency laid on top of the background alongside all the other layers it’ll be brighter than the background which has way more plastic covering it. You can disguise this somewhat with a much higher budget for the object, a version of the background with the object and one without so it suddenly snaps into the Fudd Flag only when it moves, or making the backgrounds super bright and equally cheap looking (which is why in cheaper cartoons its ironically not as obvious).
For the full explanation, go to this link: https://www.reddit.com/r/comics/comments/1p27k7p/comment/npvhut5/
