Let's say I have the following tile:

The tile has 2 food and 3 science. If I placed a farmer's quarter nearby, the science yield dissappears. I get what that means (I'm not exploiting science from this tile). However, I'm still able to use this yield later if I placed a science quarter on this tile or nearby. It's easy to forget that there's some science on that tile, and the possibility to lose yields arises.

I suggest that the yield has to be greyed out or smth, but not dissappear completely, because in fact you can still use it if you build a proper quarter nearby.