So watching some Russian media channels.
According to Ustinov's manager, they were sent in a request from Usyk's team and said they agreed to it.
https://www.youtube.com/watch?v=vbSFIfHKGLg
Might be something as simple as preferring not to chance it on the visa... last thing they need is having to find someone else, man.
Hard to say which is worse, but at least Ustinov has been in with some live bodies within recent memory, even if he lost.