Thanks!
One I just noticed is that if streams are connected via a culvert, then they show up as connected via colour & length, but it shows a gap at the connecting point e.g. https://amandasaurus.github.io/osm-river-basins/?tiles=planet-waterway-name-no-group#map=18.71/-28.0914792/153.3986749 & Way: Camp Creek (653704349) | OpenStreetMap
Edit: But then in other spots they do & don’t connect! https://amandasaurus.github.io/osm-river-basins/?tiles=planet-waterway-name-no-group#map=16.85/-28.014578/153.348496 is connected under the railway, but not the motorway, but Way: Bunyip Brook (193407963) | OpenStreetMap & Way: Bunyip Brook (193407995) | OpenStreetMap show they have identical descriptions etc?