No idea why I made it became a multipolygon, i probably used the auto-suggested fix in id, no idea.

referencing geofab isn't providing an actual learning resource though. Please be mindful that other users do not know about geofab, and do not know how to use it, so therefore blindly referencing it as a sole be-all, and end-all does not provide anything of educational note.

Osm has a wiki for a reason.