on arm64:
root@3cf1ebcf6277:/# apt install -t llvm-toolchain-bullseye clang  
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
clang is already the newest version (1:11.0-51+nmu5).
0 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.

on x86:
root@149a19ca001b:/# apt install -t llvm-toolchain-bullseye clang  
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
clang is already the newest version (1:18.0-58exp120230727113808.19).
0 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.

Can this be a issue with the meta package?

I'm able to install the nighly toolchain if I specify clang-18 explicitly.