Star Rail uses a 28-byte KCP header instead of the stock 24: a 4-byte session token follows conv, both little-endian. The vendored copy is patched for it:

IKCP_OVERHEAD 24 -> 28
token field on IKCPSEG and IKCPCB
ikcp_encode_seg / ikcp_input write and verify it
ikcp_settoken(kcp, token) to set it after ikcp_create

Handshake packets carry conv/token as big-endian u32; the KCP header carries them little-endian. That byte reversal is what the real client expects.