There’s an open issue on the Flathub repo (Aarch64 build · Issue #63 · flathub/com.bitwarden.desktop · GitHub) to support an Arm64 build as a Flatpak.
This was available up until version 1.7.0 but since then only x86_64 builds have been available. I appreciate this is an unofficial distribution channel but if it is possible for Bitwarden to help with this that would be greatly appreciated.