At a certain point i have (example) a library and a academy together in a spot. If i want to built a universaty, there is an option (most of the times the best option science wise) to built it on the library and academy. If i choose that spot, one of the other buildings seems to dissapear. What's up with that?
Also the obsolete buildings lose almost all of their yields (adjacencies, slots, counting as a quarter, etc.) only leaving some small base yield* and a maintenance costWell obsolete buildings are replaced with newer ones so If your Library got replaced by University that is intended,it's called overbuilding. Or do you mean the other building also inexplicably disappeared?
Since an Urban district can have only two buildings it makes sense to remove a useless one to make room for a useful one.
*The library does still produce a tiny amount of science, but not much.