This will probably be a quick one - on the server, it is a cubic chunks world (for example, no bedrock), but it still doesn't let me build above 255. I'm guessing this is a thing that was added client side but not server side. Thanks!

(oh, you don't have to answer this, but what's the world type called internally, so I can spawn it from server.properties without drag/dropping my save file?)

The world type is the generator type. Change DEFAULT to VanillaCubic or CustomCubic or FlatCubic

As for the 256 limit - can you create blocks above that limit using commands? I remember there was an issue with building exactly at y=255. And it may be the build height in server.properties not being changed.

Ohhhhhh there IS a build height in server.properties! why does that exist >.<
Well, thank you!

Note that build height in server.properties is NOT the same as world height stored in NBT. Build height is the height accessible for players to build with, trees for example can still grow higher, but it will be impossible to break them for players (that's how it works in vanilla)