According to RFC 8446 section 5.1
"""
- Handshake messages MUST NOT span key changes. Implementations
      MUST verify that all messages immediately preceding a key change
      align with a record boundary; if not, then they MUST terminate the
      connection with an "unexpected_message" alert. Because the
      ClientHello, EndOfEarlyData, ServerHello, Finished, and KeyUpdate
      messages can immediately precede a key change, implementations
      MUST send these messages in alignment with a record boundary.
"""

The TLS implementation does not fail with alert(fatal, unexpected_message) when a KeyUpdate record is not on a record boundary, but this is required by the specification (as a key change happens immediately after a key update record)