They likely have a white list of browsers, not a black list, so new browsers may simply not be supported until they update the list.
Edit: And the new Edge version likely identifies itself differently.
The change in useragent would indicate that if they didn't Add specifically to exclude it, it should detect as Chrome.
It sends 3 different useragents in different circumstances for some reason. Edg, Edge, and Chrome. This case is definitely the Edge team's fault.
The circumstances are domain-specific and it's been clear from the beginning that "Edgium" has (consistently) only been sending `Edg` to youtube.com from the beginning of the public release. Since the site in question's behavior changed, it likely wasn't the Edge team's fault.