You cannot place blocks above Y level 319, despite the build height limit reached message saying the build limit is 320 blocks, as shown in the image below
The message saying 320 may also be an error, since there are 64 more blocks below y 0, though this may be intentional
Before it did say 256 when in fact it was Y=255, but that made more sense when Y coords and height matched. That is no longer the case, so it would be better to explicitly say "319" blocks (or move the world block range to -63...320 or something)