I live in Belgium, which has a complete coverage of boundary=postal_code areas. (At least in the part of the country I will now be talking about.)
Typing “1410, Belgium” in the search box will correctly link to a “postal code” relation with that postcode: Relation: 4727648 | OpenStreetMap
Surprisingly, most postal code relations in and around Brussels can no longer be found, despite they exist in the database.
Typing “1083, Belgium” will not link to the desired relation, whereas Relation: 3379420 | OpenStreetMap was expected. AFAIK, until recently this worked fine.
Also, Brussels city centre has postcode 1000 (Relation: 3379410 | OpenStreetMap) but Nominatim will ignore this relation and return postcodes from other municipalities, e.g. this incorrectly returns postcode 1060:
I inspected postal code relations: they look properly tagged, ways are ordered properly, no gaps, and areas do not overlap.
How can I see whether the problem is with OSM data or if this is a problem with Nominatim?