It looks the patch is (also) in Mjolnir, thus on the import-side of things. Could it be you've used a more recent version of Vahalla, including this patch, to import and convert the data from OSM? I guess it doesn't matter, and it's now clear that "255" means "maxspeed: none". This sign, appropriately the German sign, seems like a good indication in such cases. Come to think of it, is there anywhere else in the world, except for the Autobahn, where there's no speed limit?