Oh you tried creating passkeys on your phone? 
So my understanding so far was the following: neither the Google substitute microG nor /e/OS have some sort of storage for passkeys, i.e. without Bitwarden, passkeys created on Android are just transient and will get lost.
But now with Bitwarden as a store for passkeys, this changes. But since I don’t know what exactly Bitwarden’s statement creation of passkeys on Android is not yet supported means, it’s hard to say what actually happened in your case without digging into the source code. But since nothing has changed in terms of /e/OS or microG, maybe you’re lucky and the newly created passkey is only partially corrupt (just a rather naive assumption from someone who doesn’t really know what’s going on under the hood) but was synced to your vault. In that case, I’d sync the vault on Android, open it with the Bitwarden app on Windows, open the GitHub entry, remove the corrupt passkey entry from there, save and sync. Now when syncing again on Android, the passkey should be gone and you have a clean place to start. But that’s really just a poor assumption 