Ah nevermind, some folks explained on the chat: https://matrix.to/#/!SqvNXrBsvdGHYRyota:matrix.org/$17321799774944RWwAD:matrix.org?via=matrix.org&via=matrix.domainepublic.net&via=kde.org
It's pretty absurd that Belgium has a T class in the national system and then use it completely randomly, but I guess it's not wrong to copy this weirdness into OSM