@Riptide Welcome to the forum!
This scenario was discussed recently in this thread.
The upshot is that the check for a valid email address (which actually only checks whether the entered string contains a @ character) occurs locally in the client app. No data is transmitted to Bitwarden’s servers (or elsewhere on the internet) unless there was an @ character present in the Email Address field when you hit Enter (or clicked Continue).
You might be be concerned about copying your master password to the system clipboard, though. Every process running on your computer has access to the clipboard contents at any time, and many popular apps have been caught routinely snooping on users’ clipboard contents. Unless you happen to have malware on your device, any clipboard scraping taking place would most likely be for the purpose of tracking and advertising, but it is possible that some big marketing database now contains a copy of your master password. If that database is ever compromised, and if the data thieves figure out that they are in possession of your master password, you could be in trouble at some point in the future.