sorry, those screenshots are from OSM Americana, for `NZ:WRR` they use the ref from the route relation (which used to work perfectly because the ref was `WR`) - https://github.com/ZeLonewolf/openstreetmap-americana/blob/3b73ce2/src/js/shield_defs.js#L3617

I guess the best solution is to design a proper svg shield, then the ref tag will be irrelevant for this scenario 