HoleMan@Pi-Monitor:~ $ pihole -up
[โ] Update local cache of available packages
[โ] Checking for git
[โ] Checking for iproute2
[โ] Checking for dialog
[โ] Checking for ca-certificates
[i] Checking for updates...
[i] Pi-hole Core: up to date
[i] Web Interface: up to date
[i] FTL binary already installed, verifying integrity...
[i] Local binary up-to-date. No need to download!
[i] FTL: up to date
[i] Warning: You are using FTL from a custom branch (development-v6) and might be missing future releases.
[โ] Everything is up to date!
Yes, this currently affects all ARM builds as the configuration of fail01.dnssec.works changed causing some of our CI containers to fail to build the most recent binaries. So yes, the display is right: There is an update available but you cannot download it as the ARM binaries are not ready.