HAOS stuck at 11.5 and won't update

Installation Home Assistant OS
2

I was in a similar boat as you:

# ha os info
board: rpi4-64
boot: A
boot_slots:
  A:
    state: booted
    status: good
    version: "11.5"
  B:
    state: inactive
    status: good
    version: "11.4"
data_disk: PNY-500GB-SATA-SSD-PNF14222322350600733
update_available: false
version: "11.5"
version_latest: "11.5"

It feels like there’s something about the newer versions that changed where the versions are and the old versions don’t see that information anymore.

I was able to work around this by explicitly selecting a version:

# ha os update --version 16.2
Command completed successfully.

After a restart it’s up to date:

# ha os info
board: rpi4-64
boot: B
boot_slots:
  A:
    state: inactive
    status: good
    version: null
  B:
    state: booted
    status: good
    version: "16.2"
data_disk: PNY-500GB-SATA-SSD-PNF14222322350600733
update_available: false
version: "16.2"
version_latest: "16.2"

So hopefully it’ll also work for you with an explicit version.