We do have exactly the same setup and issue. Polycom Support is pointing us to Microsoft, as the logon screen comes from the SfB client and aguably so, the logic to prepoulate the user name field.
MS Premier Support is so far not able to help either.
Only indication that this must me achievable (although causeing too many side effects for us to deploy) is this way to make the UPN appear as User name by basically disabling modern auth / ADAL with leads to more password prompts, and removes big parts of your SSO - not what you want really.
EnableAdAL REG_DWORD 0x0
NoDomainUser REG_DWORD 0x1
DisableNTCredentials REG_DWORD 0x1
So that makes at least two of us suffering from this :)