Hi,

You seem to contribute a lot. But you did not answer to my question. Maybe you did not get the previous notification?
Never mind.

So, I look again:
* city boundaries https://www.openstreetmap.org/relation/58207
* postcode https://www.openstreetmap.org/relation/10569312

They look similar. And match: https://ville-ronchin.fr/se-reperer-dans-ronchin

You did not give more information, but I think I get you problem: https://www.openstreetmap.org/way/589655145
states:
addr:city 	Lille
addr:postcode 	59000

Indeed: https://www.gastonberger.fr/ is in Lille as an address. Why? Probably because the entrance/front office is in Lille (not far of the boundary). But most of its surface is in Ronchin.

It does not shocked me that postcode boundary include all Ronchin and this school with a Lille address.