During StartMount call GaiaIdMigrationStatus is not set for a new AD user. So it uses email as a cryptohome id:

#1 0x56c65be553f4 cryptohome::Identification::Identification()
#2 0x56c65be7b4f1 chromeos::(anonymous namespace)::StartMount()
#3 0x56c65be7b6cf chromeos::CryptohomeAuthenticator::CompleteLogin()

After that I see error in logs.
ERROR:device_event_log_impl.cc(159)] [05:02:18.289] Login: cryptohome_util.cc:131 GetKeyDataEx failed with no GetKeyDataReply extension in reply.

After that DoMount is called:
#2 0x56c65be7eff4 chromeos::(anonymous namespace)::DoMount()
#3 0x56c65be7e630 chromeos::(anonymous namespace)::EnsureCryptohomeMigratedToGaiaId()
#4 0x56c65be7fa4c chromeos::(anonymous namespace)::OnGetSystemSalt()

EnsureCryptohomeMigratedToGaiaId marks AD account as migrated so it uses right cryptohome id after that.